A watered down version/place to start: Every 100fps ='s 1bhn or
800fps ='s 8bhn
900fps ='s 9bhn
1000fps ='s 10bhn
While stating different lead/tin alloys is a good thing and casters tend to get better results using lead/tin due to the elasticity tin brings to the table then alloys using lead/tin/antimony. The lead/tin are expensive and not a cure all/plug and play option. A link to article in an American Rifleman magazine from May 1953.
http://www.goodrichfamilyassoc.org/44_Special_Articles/44 spl - may 1953 american rifleman.pdf
The bullets are 44spl's instead of 38spl's but the alloys/velocities/results will give you an idea of what they were getting with 4 different cast hp bullets and 4 different lead/tin alloys.
40 to 1 ='s 8bhn
20 to 1 ='s 10bhn
16 to 1 ='s 11bhn
10 to 1 ='s 12bhn
The original "Lyman" #1 alloy was 10 to 1. The modern "Lyman" alloy being used today is Lyman #2 5%/5%/90% tin/antimony/lead.
Not all hp's are created equal. The depth and shape of the hp will make a huge difference in the performance of the hp. Typical 38spl/2" to 3" bbl velocities run in the 850fps to 950fps range with 150gr to 158gr cast hp bullets. I've cast and swaged my own hp bullets for the 38spl/357's for decades. Did a lot of testing them in 2"/2 1/2"/4"/6"/8"/10" & 12" bbl.'s in revolvers and tc contenders. These are some of the last cast/swaged hp bullets I tested using a 2" bbl'd 38spl.
Most hp pins for 38spl/357 cast bullets are 1/8" in diameter. Lyman's and h&g pins tend to have round points where the cramer hp pins could have the same rounded point or taper to a point depending on the intended use of that bullet. The top right bullet is a lyman 358158 hp, it was a special order with a .156" hp pin. You used to be able to order the lyman molds with the standard .125" hp or or the larger .156" pin (.156" pins were typical of the 44/45cal molds). The bottom center bullet is a cramer "hunter" bullet that was designed for the 357 with it's small tapered hp. The bullet in the top row 4th going right usies the standard cramer 38spl hp pin which also happens to be .156".
Some 44cal hp's cast from the same mold just different hp pins/shapes/depths.
Penta points hp's ='s 8bhn 800fps to 1000fps
Large round hp's ='s 10bhn 1000fps to 1200fps
Small round hp's ='s 1200fps to 1400fps
For my short bbl.'s 38spl's/9mm's/44spl's/45acp's I typically use a 8bhn/9bhn alloy that's nothing more than range scrap that performs extremely well in the 900fps to 1100fps range. For the 357's and 44mags I use a 12bhn/13bhn alloy of air cooled wheel weights an tin added.
If it was me I'd start with a +/- 8bhn alloy and start testing in wet pack. Wet pack is bundles of newspaper that are taped together 10"/12" and then set in a cooler. Fill the cooler with water and let it sit over night. It typically take being watered 2 times, at the beginning and then then 2 or 3 hours later. Take the cooler filled with wet pack to the range and pull a bundle out, staple a target on it and start testing. Easy enough to recover bullets along with getting an idea of penetration.