That's pretty close to the formula I used on the bathroom counter, Mother's frog and what I'll continue to use until I can get to Lowes to buy some gray mortar and gray grout. Mind you, I don't measure exactly - just use a cup to dip, and if it LOOKS RIGHT, I go w/it. Don't know WHY I bought white mortar!

Don't forget, though, Janie, she mixed it because she wanted a particular color.... not so much because she wanted a mortar/grout combo. I'm all for doing what you need to do to get a job done, but grout works great as grout. It was designed to do that job. I can see mortar working for the grout, but to be mixing them just to be mixing them might end up adding more work and dollars when you don't need to.