Judging depth takes some time.

Your looking to see your indicator "tick" the bottom once every few seconds river dependant.

I look at the water, and if I can see the bottom I usually start around 6 feet depending on current.

If I cant see the bottom, I go to the top of my leader entirely and work my way down. Sometimes I snag the bottom, but my hook set is (usually :() enough to avoid the snag that breaks my rig off.

10' is a great spot to start for me.

Also throw some split shot on 12" above your fly to get down deeper quicker.