The Back of my soil suphur bag just says apply at rate of 1-2 pounds per 100 sq ft. I know just what your looking for, X amount to lower pH from Y to Z. Had it once and filed it somewhere from pre-computer times, but heck if I can find it now.

If you are working with black soil for alpines, that is still way to rich. I hope you are amending it with sand/chick grit. Rich soil will make them grow larger and wimpier than they should be.

Also, granite grit is what you want. Granite is natural more acidic.

You can buy acid in powder for for gardens. Its mainly for hydrangeas, to change their color. Maybe that stuff will help or use a vinegar drench.