How would I go about replacing them with 3-dot sights, and which would be the best for carry?

I have an idea to potentially use one of these to make life easier for my local smith since he has trouble with dovetails (he has low prices and I don't have to ship it anywhere), but I haven't talked to him about it yet.

My current problem is determining which rear sight to purchase. I've been googling for about half an hour and can't come up with a measurement, and I don't have a micrometer.