IMR4064 is a good powder for the 8x50R Lebel cartridge
May I suggest you start with IMR4895 data, reduced by 10%
IMR4895 is slightly faster than IMR4064 so such loads will be safe.
I used 42.5 grains of IMR4064, under 196 grain .329 diameter bullets, over CCI250 magnum primers in my Forestry Carbine.
This load generates slightly over 2300fps out of my carbine.
These handy little rifles can be very accurate, with proper sized bullets.
Bore diameters vary from .323in to .330in, even with barrels not showing any wear.
finding the three round chargers can be difficult, but they are available in Canada if you do an internet search.
The bore in my barrel, is .328 diameter and it's about 99%, bright and shiney.
I would suggest you slug your bore to check proper bullet fit.
Many of those old warhorses were ridden hard and put away wet. My load would be close to "starting" load IMHO.
That's fine with me, those little rifles generate a lot of muzzle flash and recoil.