Yes, 1/2 tank or a little less should be low enough. Best to use a brass drift to loosen the lock ring if you don't have the correct spanner wrench. Don't want any sparks back there...just in case.
If you want it as accurate as possible, have your original unit rebuilt. A search on FABO should come up with a couple of companies. Not real cheap though.
If you decide to go with an aftermarket sending unit, post up your old one for sale here. Someone will want it.
