Ok setting up the rear mech after fitting new cable, read about setting the b tension scores so that its 12 to 15mm gap between 42 tooth cog and the top jockey wheel, is this from the tip of each tooth??
Found setting this up a struggle but think I’m getting there

Don’t bother with the measurement, wind it in just until you get consistent shifting between the 36 and 42 (you will notice the jockey wheel hitting the 42 when going up if it’s too little)