Working backwards to get level of significance?

Hello all,

I am familiar with tables such as this for determining the level of significance, given a certain df and R value.

However, I have been searching high and low for the equation used to get these numbers with no luck.

I am trying to figure out what my level of significance would be if I had 47 degrees of freedom and a much higher correlation than the ones listed there (.9085 for polynomial trend line and .8653 for linear).

Does anyone know the equation that I can use for this?

Thanks!