Big subject covered in the Glider Flight Training Manual beginning on page 248 through page 256.

Clear the trees by a safe margin. In this case round it off to 100 feet.

Full dive brakes (type certified glider) and no wind, the glide ratio is 5:1.

This results in 500 feet plus round-out, flare, touchdown, apply brakes and stop.

Being the expert pilot you are, make it 1,000 feet. Try it at your familiar home airport.

Tom Knauff

Lots more info in the book.