For a 125 grain bullet you don't want absolute maximum velocity I'd suggest using Unique or something in the medium burn rate. Other powders of similar rate would be AA#5, Hodgdon Universal, IMR 7625, etc. Try 8.0 grs of Unique to start and that should give you around 1,250 fps from a 4" barrel and you can go up to 9.0 grs for a little under 1,400 fps.
For the .357 magnum with 125gr bullets I've used 13.8 grs of Blue Dot with magnum primer for years and this duplicates factory full power loading about right around 1,450 fps from a 4" barrel. Blue Dot isn't recommended for the .357 mag and 125 gr bullets now but some of the data listed for it went up to 16+ grains with the 125 bullet though most topped out at 14.6grs. Alliant just issued the warning a couple years ago on new lots of Blue Dot.