Chris, it is the difference in the final scores that matters, so how you arrive at it doesn't really matter so much. I notice that some groups add all the factors to their die roll to get a total whilst some other cancel out factors that apply to both. The end result is the same as the difference between the totals is what counts.
For what it is worth, what I would have done is given the French +4 for lancers charging and +2 for charging = +6
The Russian Uhlans get +4 for lancers and +2 for charging = +6
The Cuirassiers get +5 for cuirassiers +2 charging = +7
The two lancers cancel each other out, so that is a par, the Cuirassiers are +1. Since the factor a lancer gets against a cuirassier is 2 less I would give an additional +2 to the Cuirassiers.
So I would have the Cuirassiers as +3 on their die roll with the Uhlans and the Lancers as par.
Your way gives the lancers a slightly better chance as you have the lancers on +5, the Uhlans on +6 and the Cuirassiers on +7. The total difference is still the same i.e. +3 to the Russians, but split over the two units (Uhlans +1, Cuirassiers +2).
Swings and roundabouts, six and two threes, 12 of one and half a dozen of the other and every other cliche you can think of, your way worked just as well.
Cheers, Gary