I just mix it to my liking. I've never been good at the exact measurements thing!
Usually more Swheat than A&H, but that depends.
As for Swheat clumping, IMO it works well. But, other people will say different. I would start trying litters to see what you & your kitties like. I like Swheat, but I'm sure there are others here who don't. I'm using Special Kitty multi cat with crystals clumping litter in the upstairs boxes & I really like it so far! I also use Tidy Cats, I'm currently trying Hy-Vee(its a grocery store) brand litter. I like to try new things. I usually mix litters, too. Like in the Special Kitty boxes, every day when I scoop I add new litter, which happens to be Scoop Away.