That certainly should be a good one. (It's early morning and I'm pre-coffee, so please don't ask me to crunch numbers.) The bottom line on settler pumps is that the town needs to grow by two in the same time it produces a settler. Ideally, that's a 4-turn pump, but it can also be a 5 or 6-turner. With a little fudging of the numbers, you can build worker pumps, too.