My first finished garment was a size 32 (according to my full bust measurement)...... The Abby cardigan a size 28 (according to my high bust measurement) and I am still having trouble with fit. You can see the back is still too big in the Abby cardi (prior post). So what I did was cut up one of my RTW tops and put it over the pattern and this is the result..
Now the pattern below does has a seam allowance - however the seam allowance is minimal (less than a centimetre). I have traced out the RTW top and that will become my sloper to put onto any knit pattern to see sizing. This will certainly help me to choose which size to cut and any grading. I am sure that there are more professional ways of doing this - tape measures and measurements etc, but measuring is not my strong suite. Now I feel a bit more confident making tops.