In my opinion, if all other needs are met, space is secondary. Now, I'm not saying it's ok if the turtles have no room to swim and have to stack on top of eachother to bask or anything like that. I kept my 2 5in res in a 50gal until I moved and was able to give them something MUCH larger. If your plans are in the works and you know it won't take longer than May or June, they'll be fine.
Btw, if you're going to the trouble of building a pond, go as big as you can. That way if you want to add more turtles/fish/plants they'll all fit.
And yes, it really is 10gal per inch

The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler