Hi D.,
The top of the Pratomagno range starts about 40 km almost due east of Florence, then they extend southward for about 15-20km.
The basic coordinates as given by wikipedia in Italian are: 43°39′N 11°39′E
(
Pratomagno - Wikipedia)
The drive takes 90 minutes because I was calculating taking the more common 2 lane roads (SS67) to Vallombrosa or Reggello, but if you map it on Google (Florence-Reggello), the directions given make you take the Autostrada A1, with an estimated time of around 40 minutes. So depending on how you choose to get there, the time will change.
Hope this information makes it clearer!