Sam,
That works well, but that's not what I did so I may have fewer than 100 in this first box and if so I'll adjust later. This box is just the prototype.
When I made my .44-77 loading block I made 5 rows of 10 holes .640" diameter on 1.0" CTC. It's a bit bigger block at 10 1/2" x 5 1/2", but it will work well. Then I got to thinking and that's where I usually get into trouble.
My .45 caliber loading block is only 9" long and 4 1/2" wide. It has 50 holes that are spaced 3/4" CTC in 5 rows of 10. The holes were just a little too small for the .44-77. So I took that block and opened each hole up to .630". The Jamison brass I have fit that block well now and they have the largest diameter rims at .625".
Not wanting the box to be any bigger than it has to be and still hopefully hold 100 .44-77s I decided if my box was 8"+ x 9"+ on the inside and I made 2 blocks at 4 1/2" x 9" to put inside it I'd be good. The box therefore ended up being 9 1/4" x 10 1/4" on the outside.
So now with the block the 8" way I'm looking at .630" diameter holes on 3/4" CTC. Looking at it this morning I should probably have made the box 9"+ sq. inside it would have been better doing it your way with 3/4" x 12 =9" , but I think what I have will work.
The plan is to put the first hole 5/8" in from the edge of the block. Then space the next 9 holes at 3/4" CTC which will leave 5/8" to the other edge from the center of the last hole, or 5/16" from the edge of the hole to the edge of the block. Obviously I have an inch more to work with going the other way so I may have to adjust the hole spacing to fit that or not, but that has done to allow for two 50 round blocks to fit in the box.
I've got other things to do today so I'll look at this closer tomorrow.