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)
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)