Your right about using a mitt or microfiber towel and just using the same bucket may lead to scratches, but what I do is I have 1 bucket with Optimum and another with just water that I wash my mitt in.
And even with our small cars I'll use a total of 4gl of mix.

By the way I checked out your web site very impressive seems like you guys know what your doing, I have 4 cars myself plus 3 cars that belong to my kids that I love to detail, but sometimes it gets a bit much for me, I just may come to you this spring.