probably a daft question but..

[the_mitchells]

what tyre pressure should i put in? 225/45/17's (audi) on a hi top

[Aidan]

are the tyres marked with a max pressure and load rating ? If so the extrapolate your load as a percentage and try that percentage of the max pressure, it's not exact but will give yoyu a starting point, it's not easy to advise on this as only the tyre manufacturers and vehicle manufacturers have the full picture, hence the restricted recommended sizes and presures, once you start going away from this then you are in unknown territory
hope the load rating on those is high or they will be well squidgy, even the correct 205/70/14s are a bit squidgy imho, I much prefer the correct commercial tyres