You do need 100%, otherwise you'll end up with a void at the top once the plastic melts. You could experiment with intentionally designing in a "sprue" of extra material to melt and control where the void is, but I haven't tried it yet.

I've successfully used this method on a sun gear for a planetary gear box designed for a low speed, high torque application. Due to mechanical constraints, the gear only had about 1.2mm of wall thickness at a 5mm radius, and it was prone to shearing off. Remelting it in salt solved the shearing, without having to redesign anything else.