Handy tables and charts.

[Harry's Lad]

Naturally you're going to suggest Chairgun is more accurate, Dave (),

Sorry, I tried not to make it read like that. An earlier version of FillCalc used the same model as the spreadsheet and – although I was vaguely aware of air’s compressibility issues – I didn’t consider them particularly relevant (but then thermodynamics was never really my thing ). However, as demonstrated, the differences above ~240 Bar are too big to be ignored. ….

. . . but as I'm not so inclined to trawl through volumes of physics books to discover which table is more accurate, I'll let the creator of the table speak for it.

No need for any books (or trawling through same), just Google ‘Van der Waal’, it’s there for all to see. Consider the post as a simple warning that the number of fills from 300 Bar may be found to be misleadingly high.
Having said all that, the Van der Waal model is a pig to work with (assuming the linear relationship is much simpler) and the differences are pretty much irrelevant under the magical ~232 Bar level.

HTH
Dave

[Edit] By the way, did you ever wonder why 232 Bar is/was a popular working pressure for a pressure vessel? Why 232 Bar; why not 225 Bar or even 250 Bar? It turns out that 232 Bar is the pressure (for air) where the Ideal Gas approximation holds true. At pressures below that you get slightly more than you expected; at over 232 Bar you progressively get a lot less; the compressibility ensures that the Law of Diminishing Returns takes hold with a vengeance.