You really need to do a proper weight check -- it's truly an apples-to-oranges thing.
If you can't do the weight check up front, you could always do the thing that some people vetch about -- get yourself to be just at eye level (neutrally buoyant), on the surface, when vertical, with a full tank. Then add 5 lbs (if diving an AL80)
There's nothing bad/evil about this -- the air in it weighs about 6 pounds total. If you're starting with a nominal 3000 PSI fill, and your end state (back at the surface) is 500 PSI (the target for most people, for normal rec diving), you will have used up 5/6 of the air -- 5 pounds. So, neutral at the surface with a full tank, plus 5 additional pounds of weight, will result in you being neutral at the surface at the end of the dive (when your tank weighs 5 pounds less).
If you're diving something other than an AL80, the same idea applies -- you want to compensate for the delta weight, the air used up. There are some online references for tanks that tell you the buoyancy empty (0 psi) vs full (whatever PSI the tank is nominally rated for).