Well known cast bullet reloader Ed Harris has several articles about cast bullet loading, you'd be much better off searching for some of his info. Whatever OAL length you need, set first & work up from there - I'm sure everyone has heard all the disclaimer & this one's mine - don't be an idiot & jup right to top loads on any reloading data.
With that aside, with H4895, I was at 24 grs in the 7.62x39 & 30 grs in the '06. OAL's were 2.220" for the 7.62x39 (thinking of using this in an AK or SKS? - won't work), and 3.100" for the '06. They work in MY guns.
The commercially cast slugs may shoot fine, but good luck getting any expansion. I think you'll find them pretty hard.