The theoretical best latency would be something like speed_of_light_in_fiber/great_circle_distance_between_regions, both of which are pretty easy to find. The first is a constant you can look up, and the second you can compute from coordinates of each region pair.
Thats what we did as well, via wolfram alpha. I.e. we were too lazy to look up everything ourselves and just asked it straight up how long of a roundtrip it would be between two destinations via fiber. We checked one result and it was spot on. This was six years ago tho