If your friend has a good load using Varget with a 400gr bullet it is simple and safe enough in these big capacity cartridges just to raise the load a couple of grains with a lighter bullet such as a 350gr as a good starting load.

I do not know why the lack of loading data in a manual is a barrier to experienced reloaders. Plenty of loads published on the internet now for almost any cartridge that will get anyone loading, a bit different to when I started loading for my 404 where without the internet about the only advice was to start with IMR3031 at or just above the factory cordite load. Elmer Keith published a few loads for the British big bores using IMR4064 and 78grs H4831 so I just started at that with IMR4831 and worked up loads for the 400gr and 350gr bullets.

Every rifle shoots different even from recommended loads in manuals so the only way to go is with a chronograph or just use pressure signs and work up an accurate load within those guidelines.

I haven't bought a reloading manual for years, the internet has taken over that role.