To be both both blunt and brief I have been loading for many years
and own many loading manuals published many years apart and have
no idea how you came to the conclusion than in any given cartridge
there is a direct linear relationship between grains of powder and
velocity. The 38 spl and 357 mag are actually two very different
cartridges that have different optimal working pressure ranges. Some
powders have some application in both but optimal powders in each
will have different burning rates. Smokeless powders are progressive
burning and burning rate changes as pressure increases so a specific
increment of charge increase cannot yield a consistant incremental
increase in pressure and velocity. A hot 38 spl may very likely be at
a lower pressure than a mid range 357 mag load and the two will
contain different powders with different burning rates that are at
different points on their respective progressive curves. There is actually
quite a bit of data in loading manuals for +P 38 spl loads but most of
it is at pressure levels barely above standard. I don't think there is
any thing wrong with a bit of extrapolation in loading for lower pressure
cartridges like the 38 spl and your best friend will be a chronograph.