Castaway, so you are saying that I want a bull that is the width of the rear blade?
Double 30, I like the idea of using electrical tape. If nothing else, then I can experiment at the range with different shaped till I get one that is easy to see. I will try the inverted T. I was thinking a solid black diamond maybe 8 inches square.
My thinking while driving to work (66 miles to work in Texas right now........you can get some thinking done) was to figure out what the front blade subtends at 100 yards and to work from there. When I was trying to hold on the small square yesterday I gave up and held level with the bottom left corner of the target page with the brown cardboard backer for contrast. This gave me two dimensions to steady the shot...........but there wasn't any contrast.
Thanks to all.