I concur with your calculation. I went back to my scribbling, and plugged in the total sight radius.......8.1 I assume, and got the 0.22222222" number as you did. You did it more easily than I did, but in the end, agreement. Guess we both are hoping to help hit the mark.
I went on the Brownell's deal and it concurred as well with the 8.1 sight radius. Now........is that the real sight radius that is in mind?...
I flubbadubbed the barrel length by using 4 inches(stupid on my part), rather than the total sight radius, so my initial numbers were out of wack. Sorry about that, but has been a long week here and then some.