How to guesstimate correct tire pressure in the winter? I ride a rather generic Merida hardtail mountainbike throughout the year. I don't own a pump with a gauge so I usually pump my tires at gas stations (that have automatic pumps with gauges).  

The problem is the following: I pump the tubes outdoors, but the bike is stored indoors most of the time. Usually I just fill the tires so that they're a little bit softer than usual as to avoid having too high pressure after the air warms up indoors.

Is there a good method to guesstimate how much lower (in bar) the pressure should be related to the norm to accommodate for this change? Is this worth worrying about to begin with?  

If it matters, the temperature difference between out- and indoors can easily be 35°C (~95°F; -20°C outside, +15°C inside).
Ideal Gas equation: pV=nRT

With +15oC and -20oC the pressure ratio between the two is: p1/p2 = (273+15)/(273-20). Or 13% higher pressure when put in the garage. 

However that is assuming you are pumping it to maximum pressure, that the tyre can no longer (or very slightly) expand, making V (volume) constant.

That is the basics for your assumption. However, the tyre would flex and expand ever slightly. So, 5-10% would be good approximation to me, depending on how much you put into the tyre, and how big is your tyre.

Saving you all the calculation, I would suggest for 


Road tyre : 700x18-24c, 85-90 psi at the gas station (less than 10% of the maximum pressure, but you need more traction if temperature is less than 0oC!!!)
MTB tyre: 26x1.2-1.5", 55-85 psi at gas station
MTB tyre 26x1.5"+, 20-42 psi
Hybrid/CX: 700x24 to 700x40, 45-85 psi.


Just a rough guide, but please make sure you check maximum tyre pressure, then subtract  10% ( just don't do bicycle stunt indoor...) Edit 2: This is for your maximum Cold-Tyre Pressure only. Your target should be within recommended (i.e. Cold-Tyre pressure at recommended pressure and make sure +10% and it should not exceed Maximum pressure).

So I believe the answer is 10% more of the recommended pressure would not do any harm if put in the garage (checking a few popular tyre in the shop)

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EDIT: I have one tyre blown up when I put close to recommended maximum pressure, and let it exposed to the summer sun locking to the pole. It was the loudest bang I have ever heard.. and I would be extra careful from now and then when pumping an old tyre pass recommended pressure (leave alone the maximum recommended)