gas gauge reading when full but goes down quickly

I don't know of an "easy way." We know what the original resistances were................

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

but the thing is, even if you get the sender "there" the old gauge may no longer be "in cal."

But even if the gauge is OK, the "repop" senders have a reputation for not having enough "range." Add to that, you need to figure a way to "do this."

If you can get the tank near MT and near 1/2 and check sender resistance, you might be able to go from there. If I was younger, I might be interested in figuring a jig to calibrate these.