I know in our house, for everyday hand washing and brushing teeth we mostly get water near the fixtures and in a line to the towel, very little around the perimeter, and rarely beyond 3-4 inches. Our 1/2 bath has less than 6 inches on each side and spacing is not an issue there.

When DH shaves, that's a wetter affair. Same when we need to wash something in the sink.

With two sinks available, what activities might happen at this one?

Thanks for telling me about your sinks - it makes me feel better that someone actually had the experience. I think we have pretty routine use like wash hands and face (washing face probably will make the biggest splash), brush teeth, and DH shaving (but he doesn't even use shaving cream so I'm sure that won't be an issue). The sink bowl is pretty deep and 18" wide btw.

We only have one sink in the full bath, but it was very recently redone and I clean the counter frequently to keep the water spots off, so I'm fairly familiar with what we get wet :-). Plenty of counter space, but very limited wet area unless DH shaves.

I also know our floor and wall in the half bath don't really get wet, and it's mostly sink.

If you have a few days to think, just clean the counters tonight and see where you get water over the next few days. Or put down some masking tape and see if it's in the line of fire.

If there's a rule of thumb for this, i'm sure someone with expertise will come along with it!