Scotch really doesn't mix well, except for the cheap malty blends like Red Label and Whyte and Mackay Special.
Most scotch is meant to be sipped neat. The cask strength stuff you want to add water to it, but just a bit - I'd do about 1 part water for 5-6 parts scotch in the case of cask-strength bottles (around 60% ABV). You want to bring it down to 40-50% ABV.