bigjeepman: my old Lyman manual shows a range of 4.0 gns to 8.0 gns of Unique under a 158 gn cast bullet - either the swc or the rn. The velocity ranges for those charges run 680'/sec to 1250'/sec. The accuracy load for that bullet in 357 runs to about 1270'/sec over a 15 gn charge of 2400, however the accuracy load for the same bullet, but in a 38 Spl with the same barrel twist is at about 850'/sec.
Looking at the velocity range for that/those bullets in the 357, I would think you could probably get that with somewhere around 6-6.5 gns of Unique. You can also try the old Skeeter Skelton method of going half the difference between loadings for the 38 and the 357 for any particular bullet weight from a 38 Spl case if you want something snappier and don't want to use up your 357 brass. For example - your Unique charge range in the 38 Spl for the 158 gn bullet runs 3.5 for 660'/sec to 5.4 for 1000'/sec - in the 357, the range is 4.0 to 8.0. If the difference is 2.6 gns of Unique between the max loads in both calibers, then you would add half of that of about 1.3 gns to your 38 Spl max load which would put you in the ballpark of 6.5-6.7 gns of Unique.
However, the total word of caution here is that those loads were to be used in 357 caliber guns only. I used them for a while until I hit it big on a pile of 357 brass but finally got away from those loadings so I wouldn't accidentally confuse them with standard pressure 38 Specials. Other than that, I would probably go for 6-6.5 of Unique under that/those bullets in the 357 and it should give you a decently accurate plinkin' load. HTH. Mikey.
