When to use measurements instead of scale/head units (Pattern Drafting for Bespoke Menswear by Rory Duffy)

I'm sitting on a '3 button slim fit short style' for a client and the half back width specified as: "⅓ scale plus 2 inches" gives me 8 and 3/16".

The half back measurement of the client's back is 7 ⅞"

So, the half back by scale does not even include the ⅜" seam allowance, let alone ease.

As advised by Rory, I have measured the half back off the client's favourite suit, which is 8 ⅝". If I add a ⅜" seam, I get 9" for the draft.

Fine and dandy.

However, once I use the client's measurement, the draft's proportions are out of whack, cause the additional 13/16" come out of the forepart, specifically are now missing between pt. 20 and pt. 21

This makes me think I should rather go with the scale and adjust the back width later-on. But I'm at a loss how to go about it tbh. I cannot just add it on CB, neither on the line from pt. 5, which would disturb the run of the sleeve head, and I also cannot make the shoulders any wider.

Help? 😵‍💫

As reference, I'm working with the following set of measurements:

Scale = 18.5 4" Drop

Chest 37

Waist 33

Seat 40.5

Height 74

Head unit 9 ¼