During WW1 Zeppelins began their missions with Hydrogen cells at 33% full. How did this provide enough lift to get off the ground? During WW1 U Class Zeppelins had hydrogen cells that contained 55,795 cubic meters of gas.

According to Peter Kämpf's excellent, detailed explanation of Zeppelin mechanics they began missions with their hydrogen cells filled with roughly 33% hydrogen.  The rest of the volume contained air.  Because hydrogen expands as air pressure falls starting at 33% allowed Zeppelins to reach high altitudes without rupturing their gas cells.
However given the constants below I'm not sure how 33% hydrogen produced enough lift to get a Zeppelin off the ground.
Sea Level, 0 Meters:
Temp Celsius: 15°
Temp Kelvin: 288.15°
Pressure Pascals: 101,325
Gas Moles per cubic meter: 42.29
Air mass per cubic meter: 1.221 Kg
Hydrogen mass per cubic meter: .085 Kg
55,795 cubic meters of gas cells * 33% full = 18,412 cubic meters hydrogen
18,412 cubic meters of displaced air mass = 22,481 Kg
18,412 cubic meters of hydrogen = 1,566 Kg
Useful lift = 22,481 kg - 1,566 kg = 20,915 Kg
The empty weight of a U class Zeppelin was 25,750 Kg.  That's without fuel, ballast or bombs.  If useful lift was less than empty weight how did it take off?
At 5,000 meters altitude useful lift stays the same because although the hydrogen cells expand and displace more air the atmosphere is less dense and has less mass per cubic meter.  Lift at 0 meters altitude is the same as lift at 5,000 meters altitude.
Another detail I found interesting is that according to Wiki the U Class Zeppelin produced 64,750 Kg of useful lift.
When I plug in numbers at sea level that suggests that the gas cells were filled to the top with 100% hydrogen.
55,795 cubic meters of gas cells
55,795 cubic meters of displaced air mass = 68,126 Kg
55,795 cubic meters of hydrogen = 4,743 Kg
Useful lift = 68,126 Kg - 4,743 Kg = 63,383 Kg
However if the hydrogen cells were filled to 100% it couldn't climb without bleeding hydrogen all the way up.  It should be noted that at 5,000 meters the air is thin and useful lift drops by almost half.
Altitude, 5,000 Meters:
Temp Celsius: -17.5°
Temp Kelvin: 255.65°
Pressure Pascals: 49,586
Gas Moles per cubic meter: 23.33
Air mass per cubic meter: .674 Kg
Hydrogen mass per cubic meter: .047 Kg
55,795 cubic meters of gas cells at 100% full
55,795 cubic meters of displaced air mass = 37,605 Kg
55,795 cubic meters of hydrogen = 2,622 Kg
Useful lift = 37,605 Kg - 2,622 Kg = 34,983 Kg
34,983 Kg of lift is maybe barely enough to stay afloat with fuel, ballast and bombs.
But even if they did it this way when the Zeppelin descended the Hydrogen cells would be mostly empty and would fill with air causing the ship to gain weight and crash.
At 0 meters
55,795 cubic meters of gas cells filled with 66% air = 36,825 cubic meters
36,825 cubic meters of air mass = 44,963 Kg!
The gas bags always contained close to 100% hydrogen. No air was supposed to be in them. On the ground, the bags were only partially filled because of the high air density and pressure. The rest of the internal volume of the Zeppelin was filled with air which could enter and escape the hull through the many gaps and venting openings.
Compare that to a weather ballon. On the ground, the big envelope is nearly empty and only a small bubble of lifting gas (today mostly helium, but hydrogen would work even better) sits at the top. Only that the nearly empty ballon is in clear sight, while in a rigid airship the half full gas bag is hidden behind the rigid hull covering.
One cubic meter of hydrogen weighs only 90 grams compared to one cubic meter of air at 1.225 kg under standard atmospheric conditions. With the same amount of hydrogen a balloon will have the same lift, regardless of altitude, because its lifting gas will displace a bigger volume but the same mass of air as it expands with altitude. With Zeppelins, fine tuning of lift by adding dynamic lift is possible but limited to maybe 5%-20% of the ship's mass (more for smaller and faster ships and at lower altitude). By growing larger, the mass to volume ratio of Zeppelins decreased, so the latest WW I ships could be built light enough to fly with their gas bags only one third full.
Civilian airships rarely flew higher than 2000 m high and had to circumvent higher mountain ranges because they filled their gas bags to maybe 80% on the ground. Flying higher was not considered worth the reduction in payload and fuel that would result from less lifting gas.
Compare that to the wartime Zeppelins. They were in a continuous arms race for higher altitude with the British aeroplanes which were sent out to chase them. And until 1918 the most modern Zeppelin designs  were ahead. The limited altitude of older ships also meant that many were scrapped after one or two years of service. The table below shows the maximum altitude at which the gas bags were filling out the available volume of the ships. The list below gives the maximum altitude of the best ship of each series and the density ratio shows the degree to which the gas bags could be filled on the ground to achieve that altitude. Note that they still carried tons of water ballast and ordnance when flying that high.

LZ 112 tried with speed what could not any longer be achieved with altitude: It had 7 more powerful engines where older ships had only 5 and could cruise at 131 km/h as opposed to the 100 km/h of the older ships. The diminishing returns of flying higher and the stress of spending close to a day at -40°C and without a pressure cabin or a pressure suit made higher flight altitude impossible. What also held progress back was the size of airship hangars: Since LZ 62 the maximum diameter could not grow beyond the 23.9 m of the last wartime ships, so their volumetric efficiency went down when they grew larger by adding length.