XVest makes vests with 1lb weight.increments. I have one that works well and.would recommend getting whichever you can afford and just putting the amount that you can handle in it and building up.
I don't think percentage matters so much, and it would be very general. For example, last year I weighed ~140 and used a 40lb vest for pullups/pushups. This was almost 30% of my weight but if you weighed 210 you would so a 65lb vest which is impractical to start with IME.