There are some online spreadsheets and java applets out there which will help you out but I don't have my links here in the office (it's lunchtime).

What engine are you using and in what type of application? I'm a old control line flyer and have always used either all-castor or a generous blend of synthetic/castor. Even my modern OS ABN engines (such as the FP or LA) like 22% total oil. The 17% or 18% RC fuels just don't work well for that application.

When I fly RC with a ABN or ABC ball bearing engine, I like at least 18% total oil with at least a 80/20 synthetic/castor ratio. Wildcat Premium Plus fits that requirement perfectly.