Garry it's the alignment of your center of mass equally over the two skis. Evening the height goes a long way toward evening that out.
I added some lift under one boot with positive results...much more relaxed and efficient skiing and I don't get "torqued" off center in pow as much.
Go halfway first (5mm) and see how you like it. Your body has probably compensated for the difference to a degree so if you even up the length it could be too much. Another reason to go halfway is I've found the lifted boot causes a bit of unwanted (or more precisely uneven) hookyness in that ski.
I've also realized my body is twisted a bit from the leg length difference. My left hip is way open, my right is a tad closed. This makes left turns hyper-easy to engage while it's tough to get up and over my left ski to engage a right turn. What helped even more than the lift was tweaking my salomon bindings (with dual toe wing adjusters) to the side a bit so my left foot sits a bit duckfooted and my right sits a bit pigeon toed. This evens everything out.
I agree with X-eastcoaster. When correcting for a leg length differential, use half of the difference as a starting point when possible. Have your customer/client/patient ski and see if he/she is compliant with the change. If he/she is, go from there with possibly more lift.