Reference is 85 dB, but the point is to get the subs at the same level relative to each other, not some absolute value. Then let the AVR (or whatever) set the levels as it normally does. The catch is that you need to loop back and check the AVR's levels to make sure you haven't max'd out the trim range, top or bottom. If you have, or are very near, then you should adjust the gain control on both subs equally (change them the same amount) to bring everything back into line. What I have done is to run the AVR's cal, tweak one sub so it is pretty close to the middle (say +/-3 dB), then do the gain cal by tweaking the "other" sub's gain control to get the subs gain matched. At that point put the subs where they should be, run the AVR's cal, and see where you end up. If one sub is too high or two low, adjust both subs' knobs equally to get the trims in range.
The times I have seen this procedure fail is when a person puts one sub say at the front wall and the other right by (or behind) the MLP, or (worst I have seen) one at the front wall, one at the back, and one at the side in a large room, so there is a dramatic difference in level at the MLP. That may make it hard to find a place where one or the other is not at the trim limits.
FWIWFM - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley