How can I correctly calculate fuel for a Cessna 152? I'm learning to fly in a C152 and I've found the fuel calculation to be a little ambiguous. Based on information from my instructor, Internet and the Cessna 152 Information Manual (1980), I've home cooked the following formula and wanted to run it by the community for some verification. 

I hope this is an appropriate use of this forum and please excuse my newb-ness. Here's my formula:

Fuel Required =
    5.6L unusable +
    5L run-up and taxi +
    24L/hour flight (rich) +
    18L (45 minute) reserve


Based on this formula, a one hour lesson conducted in the training area and below 3,000 feet AMSL requires a minimum of 52.6 litres in the tanks, calculated as follows:

5.6L unusable + 5L run-up &amp; taxi + 24L flight + 18L reserve = 52.6L total


Or 26.3L in each tank. Since we're measuring fuel by the surprising method of dipping a stick in the tank, I'm realistically looking for around 25L per tank, and maybe topping it up to 30 per tank to be on the safe side.

Is this correct and reasonable?
The short answer is: Yes, your calculation is reasonable.

Fuel consumption values are not an exact science, they are average values and rounded in many cases or based on pilot experience. Using 24L/hr in your example for traffic patterns is the right assumption, as you will most likely remain in rich configuration. Even if you lean a bit within the pattern, you would not gain enough fuel efficiency to offset these 24L/hr. 

Your 5L run-up and taxi obviously depends on the airport you are based at or use the formula at. If this value was your calculation for a smaller airfield, you might need to adapt your formula when flying to larger regional airports or airports with high-traffic, where your taxi time could be significantly longer.

Be reasonable however with topping up your values. Going from 52.6L to 60L is the equivalent of 5.3KG, which you will need to consider in your weight and balance calculation.