When drilling a disc, there are a couple important points to consider. First, for improved cooling, you want to create more surface area, and you will create the most by making the holes the same size as the disc is thick. Here's a thread on it .....
https://www.xs650.com/threads/drilled-disc-hole-sizes.59637/#post-692555
The second important point is you want the holes to cover the whole pad swept area. To achieve that, the edge of one hole should overlap the edge of the next one like so .....
Unfortunately, that old disc from Griz's stash doesn't meet either of those requirements. One more thing - to keep the pads scraped clean, you want to leave the holes sharp edged (don't countersink them). They look countersunk on that one. So, to me anyway, that disc is nothing more than shop wall art now, lol.
https://www.xs650.com/threads/drilled-disc-hole-sizes.59637/#post-692555
The second important point is you want the holes to cover the whole pad swept area. To achieve that, the edge of one hole should overlap the edge of the next one like so .....
Unfortunately, that old disc from Griz's stash doesn't meet either of those requirements. One more thing - to keep the pads scraped clean, you want to leave the holes sharp edged (don't countersink them). They look countersunk on that one. So, to me anyway, that disc is nothing more than shop wall art now, lol.