It needs to be pretty tight.
If you don't mind taking the tranny apart again:
Pull out the diff, lock the outdrives with a couple hex wrenches, and try to turn the gear. If you can turn it(making the diff slip) it is too loose, tighten it up until it is nearly impossible to make slip.

When you re-install it, make sure the slipper is slipping before the diff.

Hold one wheel and the spur gear, and turn the other wheel. Can you see the topshaft and brake nut moving? If so, good. If not, loosen the slipper until it slips instead of the diff.

It'll probably loosen up some when you run it. So watch out for that. You NEVER want your diff to slip, because this is what wrecks your diff. The diff slips, the balls heat up, and melt themselves into the plastic gear.