From his website:

The .375 Renner is a cartridge designed specifically for the Ruger No.1 and hunting North American big game. It provides generous taper of the case, a gentle shoulder, and robust rim, while having capacity to hold 60 grains of IMR powders under the bullet. It easily duplicates ballistics of the .375 Whelen, or the more recent .375 Hawk/Scovill...

While patterned after the classic English singleshot cartridges of yesteryear, the .375 Renner is a brand new design and is based upon the .45-90 Winchester case. The .45-90 case is simply necked down within an appropriately shortened .375 H&H sizing die. It’s that simple. The result is an amazingly flexible cartridge capable of taking anything from marmots to moose and it will do it all with less stress than produced by the big .375 magnums.

Although the .375 Renner is a wildcat cartridge, it’s simple to make and load and even the novice hand loader will master it quickly. Yet, it offers new horizons of ballistic exploration for the most advanced hand loader - and we haven't even started exploring the potential of blackpowder in this case.

I see he has added a link with a cartridge drawing and loading notes, also some Quickload calcs.
Load notes

Quoting from that page:

The loads listed are intended for the .375 Renner English Stalking Rifle in consideration of its 7-pound weight. Heavier rifles can be loaded to higher levels of performance, but beware that recoil increases significantly, too.
For the most fun and economy with your .375 Renner English Stalking Rifle, keep bullet velocities below 2200 feet-per-second and save your shoulder for serious hunting loads over 2200 fps - IF needed.

A summary of QL loads shows:
Sierra 200 FP at 2418 fps (moderate load to not over stress this bullet)
Speer 235 at 2400+ fps
Speer 270 at 2267 fps
Hornady 300 RN FMJ at 2136 fps.



Gunnery, gunnery, gunnery.
Hit the target, all else is twaddle!