Even though I have a Butternut vertical in the antenna arsenal, this .pdf file from SteppIR, is one of THE best articles about radials that I have ever read. It's in "plain ol' English" and doesn't require you to have a Doctorate in RF Theory to understand it. Just thought I would share!
I currently have about 50 radials under my Butternut. Looks like I could stand to add some more next Summer - although I'm currently close to the point of "no extra benefit". Looks like with about 25 more, I can get a bit closer to that 90% efficient point. And "next time" (if there ever is one), I will seriously consider NOT ground mounting my next vertical.
72 de Larry W2LJ
QRP - When you care to send the very least!