Here's the hottest load for the 243 with 100 grain bullet that I 've seen used.

48 gr H4831 powder(Hodgson)
Velocity....3150 fps
Energy......2200 ft lbs

That's a MAX load. I'd be inclined to start at 45 grs, working up to no higher than 47 grs.
The above data is based on a 22 inch barrel. Very accurate load.

No experience with the 358 win

The free issue of Hodgdon Basic Reloading Manual 2005 has some great loads for both of the rounds you ask about. The same info also comes in a magazine that Hodgdon has published for about $8.00. Both are usually available at your local reloading supplies supplier.
The nice thing about these is they show the pressures of the various loads. Typically, the slower the powder the lower the pressure and the higher the velocity. There are a few exceptions.
I myself like to fill a case as full as possible. Not less than 90% and keep the pressure level in a safe range. When you start loading to maximum. The pressure can spike to very dangerous levels. I've never understood why anyone wants to make a small or mid size cartridge perform like it's bigger brother. If you need something that has bigger performance. Get a bigger gun.
I've found that H1000 in the 243 and Varget or BL-C(2) in the 358 are ideal. The pressure stays under dangerous levels and the velocity is within the maximum of the cartridge. Then again, I like those Hodgdon powders.