Deep enough that the flywheel doesn't hit it, and not so deep that it rubs on the gearing Sorry, lol. I don't know the actual depth. If you're real serious about it, the install tool PN is 1128 890 3900.
Be careful - there are TWO different FW nose lengths on the 064.... The first few years had a longer nose. Put some magic marker (or whetver) on the FW nose, push it on by hand and make sure there is no transfer to the seal.