Do you have thermal measuring instrumentation that would enable you to sense the difference in temps , while using say,a 3x8mm slot, from that obtained with a 2.5x8 mm slot, or any other size, for that matter....
without such expensive testing equipment, what would the point be in trying to get the perfect size.... How can you know....
my point is.... it is not rocket science.... take a mad stab at a size, you might choose the perfect dimensions right off the bat
is how I designed my first blocks....