Herco is a very old shotgun powder slightly slower then Unique.
30-40 years ago, there was a lot more handgun data available for Herco as there were not a lot of good handgun powders to choose from then.
Since the .40 & 10mm are relatively new calibers, Herco had fallen by the wayside for handgun use by the time they came out.
I don't know of any reliable formula to convert jacketed data to lead data.
Burn rates vary somewhat by caliber and bullet weight, so any formula would be a WAG at best.
For instance, old Alliant data in the 357 Mag:
158 grain Lead SWC.
6.8 Unique gives 1,295 @ 33,900.
7.9 Herco gives 1,365 @ 33,900.
With a 200 grain Lead RN:
6.0 Unique gives 1,105 @ 33,900.
6.1 Herco gives 1,105 @ 33,900.
As you can see, with the heavy for caliber 200 grain bullet, Herco reacted quite differently, and reached peak pressure much sooner then with the lighter 158 grain bullet.
In all, substuting one powder or bullet types data for another is risky business at best, even when one is slower burning powder then the other.
rc