Your draw length will vary with the pondage of the bow you are shooting.
For me , to calculate my optimal draw length, I pull a fairly light bow back to where my shoulder/back feels comfortable.
I measure that (as above) and then work on my regular weight(s) to achieve that draw length.
I was off trad archery for a while, and when I stared back up, could only pull 28" at 60# with some degree of effort and discomfort.
I now pull 72# at 31 inches, which is very comfiortable for me.
Botopm line:
Determine your "comfort zone" and work towards it.