The useful thing for the double sided lance would be to make its side-swings do a substancial more SLASH damage amount. Afeter all, that's partly what a double sided lance is made for ... slash with the blade of the tip (big one). So i would say that 30 slash damage for side swings would be ok, provided that it is a 130 length polearm, it will be balanced (faster than most big polearms, small amount of sideswing slash damage, so balanced)