Here's some factors to consider
for sorting out this recipe:
Current published data includes-
1. AA 3.2.2 and AA2004: 6.8 gr. This is based on standard 38 Special psi criteria--3.2.2 lists this at 16,300 under a 125 XTP
2. Speer 14: 7.1 gr. for any Speer 125 jacketed. Pressure criteria--"under 17,000"
3. Speer 14: 7.8 gr. for any Speer 125 jacketed. Pressure "under 20,000."
Current SAAMI specs call standard 38 Special to be 17,000 and 38+P to be 20,000. (See RMc's comments in post 9--3 down.) Preceding standards ran "standard" to 18,500 and "plus-P" to 20,500 (IIRC).
4. Then there is the (European / Metric) CIP standard: CIP does not differentiate for 38 Special--its one standard is (the equivalent of) <=21,750.
One "rule of thumb" given to me by a (powder) manufacturer ballistican is to take standard load recipes and add 5% charge weight for Plus-P and to add 9% charge weight for CIP.
Note that using the Speer 14 standard 38 Special recipe for 125 jacketed bullets--7.1 gr--and adding 9% gives 7.8 gr.
All of these calculations are based on a typical LOA, or BSD / Bullet Seating Depth. Currently, I am testing for the effect of Long and Short LOAs at near-max pressures. Short LOA tests use the bullet seated to the absolute maximum depth but still within the cannelure . Needless to say, BSD varies depending on the bullet design--and maximum depth seating will spike pressures.
For this particular topic, it would appear that 7.8 gr. is probably the upper limit of max pressures for the 38 special--but that LOA / BSD might justify the 8.1 charge. Depending on the revolver, you may or may not want to shoot it routinely.
FWIW, I find that #5 is an extremely useful 38 Special powder--you can load it mild to wild, so to speak, and it performs consistently. Lower charges / pressures tend to be dirtier, and overall I consider the powder to be dirtier than (for example) Ramshot True Blue, and it usually doesn't produce real low SDs except at near-max or max pressures--but it does work well overall.
Jim H.