80 is a perfect level to make that transition. Many people make the mistake of trying to chin when they are like 50-60 ranged, and they end up wasting tons of cash.
I went from 78-99 and it was great, so I would reccommend having a minimum of 70 ranged, and a maximum of 80, before you start chinning (depends on your budget as well). Naturally, the exp p/h will increase with your ranged level.
Just as a side note, it really helps tremendously if you have the Reverance aura and a set of void knight robes. It helps your prayer drain slower & the void robes add a massive hidden bonus to the accuracy and damage inflicted by the chins, which increases your exp per hour quite a bit.