Castagnatello is between Castel del Piano and Seggiano.
Once you reach the SP160 road (the road that connects 
Castel del Piano to Seggiano) exit at the km 19,7
(signposted on the side of the road as 19/VII).
Take the dirt road, then take the first on the right and you will soon find
Castagnatello's gate on in front of you.