4.5 grs. of Unique with a 158 grain cast bullet has been a classic standard pressure .38 Special load for as long as I can remember. The Lyman manual shows it getting right around 16,000 CUP with three different cast bullet styles.

5.1 to 5.3 grs. depending on the bullet style gets around 18,000 CUP and is shown as +P.

Gunnery, gunnery, gunnery.
Hit the target, all else is twaddle!