Before you try molding pyrex, you should consider the difficulties. 1) Precise temperature control is required, esspecially for the annealing. 2) The temperatures needed are very high and not easily controlled in the average kiln.

Why dont you consider slumping a thin disc of pyrex to the desired radius instead. The temperatures required are lower and easier to control although precise control is still required for anneal. The temperature required to slump pyrex is only 1500 degrees.

Experiment with ordinary plate first to see how the process works. Now the temp. required to slump less than 1200 deg.

Before you try any of this though, learn more about glass and especially about annealing.