First post here from a UK based racer (Hey!) Setting (hot) tyre pressures perfectly is a challenge we all face, whether you're using nitrogen, dry air or straight air.
If you don't have specific tyre data then you might be interested to use the following table as a starting guide:
These are target hot pressures i.e. what to aim for when the car is on track.
14 – 18
1.0 – 1.25
Very Light Racing Car < 800kg
22 – 29
1.5 – 2.0
Light 800kg – 1000kg
24 – 32
1.6 – 2.2
Heavy 1000kg – 1400kg
28 – 40
2.0 – 2.75
Very Heavy > 1400kg
37 – 40
2.5 – 2.75
As you can only set the pressures when they are (cold) in the pits, you typically have to set them lower so that when out on track you hit the target pressures.
Each track of course has its own corner characteristics. That mean tyres warm up on each corner differently.
So it then becomes a kind of double guess - guess what hot pressures you want and then guess what cold pressures to set them at prior in the pits, given the characteristics of the track.
It is however entirely possible to set your pressure correctly and reliably each time - even considering changes in air and track temperature if you really get into it.
I've found tuning for a scaling factor, rather than a fixed pressure offset, works pretty well.
So say your target pressure was 27 psi front and 26 psi rear. Then say your track is clockwise, with some low speed right hand corners. You might feel that layout may put a lot of energy through the rear tyres, and more than the fronts, and therefore increase the pressures more at the rear.
Typically people might try a 1 or 2 psi stagger around the car. Instead you might try applying the same logic to the scaling factor.
Scaling factors are typically between 1.2 and 1.5.
So in the example above, you might choose scaling factors of:
Front Left: 1.25 | Front Right: 1.3
Rear Left: 1.2 | Rear Right. 1.25
You then divide the target pressure (27 psi) by the scaling factors for each corner. This then gives you your target initial pressures. So:
Front Left: 21.6 | Front Right: 20.8
Rear Left: 21.7 | Rear Right. 20.8
The next step is you simply go out, do a run. Record what the hot pressures actually end up at. Then tweak the scaling factors accordingly so that next time you go out you can guarantee hitting 27 psi front & 26 psi rear. Or in fact, target any pressure you like.
I've actually written a bit more about this on my blog (link below) that includes how to cope with environmental changes and a free calculator too but really this is the core principal.
Hope that helps.