I think ultimately I have to "challenge" this, depending on the poker table.
If you have a table on the smaller size, like a 7'er (say, 84" x 45"), following this guideline could bite you. The "rule of thumb" for elbow space is 24" inches. Following this guideline on the table size mentioned gives you a slot at 20.884" and another 2 at ~21.278" of rail perimeter or "elbow space".
These 3 slots break the bank.
I'm still working on measuring the other slots, but as the table is rounded those restrictions don't matter as much as humans are more "flat" and not "concave" to match the circle.
Anyways, what is to be done? I think the answer might be dependent on table size...if you are looking at ~24" per player, you should use the outer rail to place cup holders. If you have plenty of room, then shrinking the "flat sides" in to provide more playing area to the "corner boys" is probably fine.
Thoughts?
I have joined you in this cup holder placement hell, LOL. Victims of our own (mild?) OCD.