A bell curve for Zenkais seems appropriate. They start small, get big near SSJ requirements, then shrink again. I think Movie 2 Goku's power of 10,000 is probably meant to reflect his post-Saiyan Saga Zenkai power when he took the senzu at the hospital. I'd have to agree with Papasmurf, cap off Goku near 600 and Raditz near 1800.