Never understood why manufacturers measure shirts from the middle of the back to the sleeve. It's only vaguely accurate, but who wants shoulders to hang off your shoulder. Hence I never buy OTR shirts. They should give shoulder and sleeve measurements and go from there. Imagine OTR makers selling shirts by collar, shoulder and sleeve measurements.
How tall are you? I'm 6'2" and my jacket sleeves from outside shoulder seam to cuff are usually around 25.5".
I'd say entry level custom jackets run around $1000 in USA.
Because the placement of the shoulder seam varies, and where it falls on any one person's shoulder varies based on cut and the person's shoulders.
Giving a shoulder measurement too would be way too much. If you have to vary three factors, you have to stock an insane amount of shirts for something that won't make a difference to many.