This link may work and then you could download an Excel spreadsheet (use Excel or Google Sheets (free).)
https://docs.google.com/spreadsheets/d/1GMtju5P-wOhImg8GdLiv3qkBMUTnsP9e_NdsWYw2IWU/pub?output=xlsx
When open, you can input your desired inflation, say 38psi, and the current ambient...
I just had my first run-in with the rear wheel on my '84 R80RT. I can't believe how close the valve stem hole is placed to one of the wheel struts and how little clearance there is for an air pump head. I'm used to using a good quality stand-up bicycle pump, but the head on that won't approach...