I don't know that you'll find an easy way you can do this in your head. It's definitely too complicated for me.
https://sciencing.com/convert-percen...s-8093249.html I wonder if it's often necessary to do this. It might be easier to just get used to dealing with both measurements. Sort of like when we talk about bike sizes in inches or cm. Most folks know their size either way w/o actually doing a conversion. Hope this makes sense. Edit- If you use something like RWGPS you'll have consistent measurements.