Kevin....the calcs of .25 amps per square inch are figures you make yourself. If you PLAN on drawing 20 amps for instance, you can decide what efficiency you want....
.5 amps/sq inch is efficient, but .25 is more so. This is what I chose, mysefl. It's not a challenge to do.
You just decide. Okay, I'm going to pull 20 amps...and you calculate accordingly. If you are pulling 20, you need 80 sq inches of reactive area.
If you look in the other posts about that, there are detailed calculations, etc.