I think the math is not too hard if you ignore the "non-ideal" gas properties at higher pressures... (but maybe you are asking for the Real Gas version?)
"Ideal gas" trimix density:
d (g/L) = ATM*(1.429*O2_fraction+0.178*He_fraction+1.25*N2_fraction)
"ATM" is like ~200(bar) for a full cylinder, and multiply the "d" by 11 liters internal cylinder volume:
air: 2833g
10/70: 1139g
So roughly add 1700g or (3 to 4 pounds) to the 10/70 cylinder to balance it out, which can be done with a cam strap and a single lead weight brick (for a BO stage cylinder)
Bit of a sidetrack, but:
The "nearly empty cylinders" weight check makes sense for a twinset, because you can't really ditch it.
For multiple stage cylinders though, it is optional, or even dangerous to do an "everything empty" weighting. This could potentially result in serious overweighting that exceeds the buoyancy capacity of your typical "tech" wing.
If your wing is already half full or more because you've weighted the rig for the rare situation of having several empty cylinders attached to you, then how much wing capacity is left for a flooded CCR loop or drysuit flooding scenario? Not much!
Rather than being overweighted combined with several negative cylinders that will normally remain full, it may also be possible in an emergency to jettison or exchange empty bailout cylinders one by one--they are no longer of any use to you!