According to MMS, that amount of lye should give you a 5% SF. I usually use MMS & have not had any problems with it.
Also, I have used raw milk in CP soap (once) before, & it was a bit fatty on the surface immediately after it set. It's been curing for just over a month now and is a bit on the soft side. I'm not sure how much of that can be attributed to the raw cow's milk. But, it leaves your skin feeling super soft. I've not tried buttermilk. I've been soaping for only about 1 year, so, I don't have a ton of
experience.
When I first started last year, I bought a pre-measured CP soap kit. It made 3 different kinds of soap, I think they were about 2lb each & I had no problems with them. I figured if I tried the kit & screwed it up, it would be me in the process of the actual making & not the measuring, it cut down on the margin of error you know? They turned out very well, so I decided to make my own from scratch. I kind of followed the protocol I was taught when experimenting with new herbs; use them singularly & learn everything you can about 1 at a time. So, I made a soap completely out of tallow and one out of olive oil. I totally loved the tallow bar, it was fabulous. I did not have the patience to let the olive oil bar cure for the many months it needs, it was alright, but too soft of course. But, since alot of oils don't make a great bar on their own, I cut my experimenting with single fat/oil bars at that point. I generally follow recipes off the internet, but, I do always run them thru the lye calc to double-check. Any good site will tell you to run even their recipes thru a lye calc before you get started.
I wish you luck Nite Hawk! Soap-making has been a load of fun for me & you can't beat the quality of the soap over commercial!