Here is data from the RCBS Cast Bullet Manual (1986)
38 Special
cast bullet - 38-158-SWC GC
dia. .358"
lube- RCBS Pistol
Powder - 2400
pistol primer = CCI 250 - Magnum Primer

Start Load : 9.0 grs. @ 926 fps

Maximum Load : 10.0 grs @ 1020 fps

Looking at the Lyman Cast Bullet Handbook 4th edition (2010) there is no 2400 data for a 158 gr. bullet but there is data for the 155 gr. SWC GC #358156 which I cast and load ... but get this , with 2400 the
Start Load is 7.5 grs @ 760fps to a
Max Load of 8.3 grs. @ 867 fps and a
+P Load of 8.8 grs. @ 953 fps .

Interesting to see what RCBS came up with ...

I think I would start low and work up slowly ... 7.5 grs then 8.0 grs ...then see how things shake out and go even slower 8.2 grs ...8.3 grs...slow like molasses in winter time ... be careful .

When fired case extraction starts getting sticky ... Stop !

Just go slow and test carefully ... and with near max - heavy loads use good , new or almost new brass . Hot 357 Magnum loads take a toll on brass .
Load Safe & Good Luck
Gary

__________________
Certified Cajun
NRA Member