Minimum speed on a board track.

There's formulae about this in the Level 2 Track coaching resources from BC.

I can't remember if it factors in rider weight though, possibly not.

Tyre choice & pressure, track temperature and track surface age might also play a factor.

I fully support M_V in saying:

I can’t recommend strongly enough against trying to find the limit of grip.

Everyone is there to have a good time, so safety is paramount.

I treat the minimum safe speed like a cliff edge, e.g. I wouldn't walk so close to the edge that the slightest misstep means I'd fall in the sea.