First of all, it is not the whale sharks that are spawning. It is the mutton snapper. The whale sharks remain in the area from around the full moon in April thru the full moon of May, possibly thru the full moon in June to feed off of the roe as the snapper spawn sometime around the full moon of those months. Now the most assured time to see them is when the snapper release their roe as that brings the whale sharks to that specific area. Other times they are just cruising around a larger area waiting for the spawn to occur. If the spawn is not happening, and being their exactly when the spawn occurs is a real long shot, you are relying on the sharks just happening to swim by you. The odds are fare that you will see one, but no one can tell you for sure.
I have read the estimated percentage that they say is the chances of seeing them and can tell you from my experience doing it every year, I think their estimates are high.