My reference Chart on the Greenhill Formula confirms you 1:25" ROT for a .411 cal. bullet that is 1 inch long.
I think more important to you is groove depth, and groove width, as discussed above. I don't think you can have your "Cake and Eat it, too!"
Shallow grooves will work better shooting conicals. Deeper grooves are needed to shoot PRB accurately.
Narrow lands may work with lead bullets that have grease grooves and are lubed, while wider lands may work better if you paper patch the bullets.
There is no doubt by ME that protecting the base of any cast bullet with some kind of filler or wad that seals which ever groove depth you use will produce greater accuracy. My father began experimenting with base wads( in cartridge casings, using black powder, and smokeless powder loads) when I was a kid, helping him reload, and then spotting his targets at the range for him. Since I became involved with BP shooting myself, I have spent way too much time trying to find some "happy medium" where the same ROT and rifling will shoot both conicals, and PRB well.
I don't think the beast exists, but I am talking Target accuracy now, not hunting accuracy. You will get lots of opinions here-- along with a few harsh arguments--- and elsewhere, unless you define what kind of Accuracy you are looking for with both kinds of projectiles. The Chunk Gun shooters in this Country are going back and forth between using a .40 caliber rifle, and a .45 caliber rifle in their sport, but that calls for PRB, and targets are shot at 60 yards. You gets all kinds of " discussions and comments" in those circles about ROT, velocity, diameter of ball, patch thickness, lube, or whether to shoot the Teflon patching, etc.
The only other BP target shooting group that has more heated arguments seems to be the Black Powder Cartridge Metallic Silhouette Shooting Association members, where those longer conicals( Longer than 1 inch) are used. Steve Garbe, and Mike Venturino have published a Black Powder Cartridge reloading manual, that is chock full of information on what works, and what doesn't( including base wads). If you are looking for the optimum ROT for any conical I would recommend spending the money to buy that book, as it gives sound advice and some data that can also be used in MLing targets rifles. For instance, ( and the reason this came to mind to suggest to you) many of the BPMSSA members are using the .40-65 cartridge using bullets, and some are using Paper patched bullets. The Garbe manual has some good loading data for the .40-65, which may help you in developing a load for your .40 cal. MLing Target rifle.
Good luck with the project, and keep us posted, Please. :hatsoff: :hatsoff: