The OP has two basic questions I believe -- the First one concerns the fact that he has determined he uses more gas than his GF. All I can say is "Welcome to the club." You will find that almost all males use more gas than females -- assuming relatively equal diving skills. You just have to get used to it and then plan your dive times based on your consumption rate. This is one reason why I dive an HP130 and my wife dives an LP95 -- and we come out just about even at the end of a normal dive.
The other question is the one to which most (every?) person has replied, "Don't rely on your GF's computer, use the tables." Which, of course, IS the "correct" answer and perhaps the safest answer. However, perhaps a touch of reality is also needed....
Someone wrote:
If you're diving using your buddy's computer, you have to stay level with or above her at all times, and on all dives. Diving deeper than the person with the computer means that it's NDL calculations are understated and therefore meaningless for you.
Come on -- The "science" of Decompression is no where close to being that precise -- or as one SB member has as his sig line -- Decompression Theory is measuring with a micrometer, marking with chalk and cutting with an ax.
The OP says he generally stays from 1/2 to 1 meter below his GF (note to OP -- learn to stay even with your buddy because it makes YOU a better buddy since it is much easier to see your buddy if she is next to you rather than above you). Do any of you really believe the various Decompression algorithms are really that accurate that a 1 meter difference in depth is important? Yes, IF the OP continues to do his NDL "planning" using his GF's computer, he really needs to stay even, or above, her to make sure that his deco obligation is close to hers.
I'm NOT saying the OP should do his dives relying on his GF's computer. I am saying that there are thousands of dives every day that go off without a hitch with the "ducklings" following a DM and relying on HIS computer or knowledge.
OP -- you are a certified diver -- be a thinking diver and dive safely.