I need to replace several sections of torn-out wall and ceiling in our house, each section about 25 sq ft. The original wall was comprised of 1/2" gypsum board with a 3/8" plaster brown coat and a final 1/8 skim coat. At changes of plane they used metal lath reinforcement. I am in Pennsylvania and the house was built in the late 1940s, if this style is unusual in the UK. I have been told it was a transitional approach, between old-style plaster-on-wooden-lath and modern drywall.

In places where the original wall was torn out cleanly, I've been able to use drywall, furring the framing members so the drywall is flush with the original wall. But in places the tear-out left a jagged plaster edge, and I've not been able to clean up those edges without ruining adjacent sections, especially at changes of plane in a stairwell. Replastering may be easier than drywall there.

I've watched quite a few plastering videos but none of them showed the mixing proportions for the scratch and brown coats, and none started out with bare studs and joists. So my questions are as follows:

Will tarpaper stapled onto the studs/joists followed by a self-furring diamond-metal-lath tacked to the face of the framing members be a proper substrate for a 1/2" thick gypsum plaster "scratch coat"?

What is the proper ratio of gypsum plaster to sand for the scratch coat? Two parts plaster cement to one part sand?

After the scratch coat dries, can I apply another 3/8 inch "brown coat" layer?
What is the proper ratio of plaster to sand for the brown coat?

Thanks