While I have not loaded any rounds for the M21 and the 165 SST is as stated "new", (we have loaded plenty of the 129 SST 6.5mm rounds) I'm interested to try some of these 165s in my 30-06. I use the H4350 as my powder for my 30-06 Rem 700 custom barreled rifle, 56 grains in once fired Remmy brass w/ CCI 200 primers, top w/ Sierra's 175 MatchKing and for this rifle/round combination, believe you'll get better results using the 4350.
However if Varget's your choice, try starting w/ 43 expecting to go to 46(C)grains (which in a 24" 308 & a COL 2.800" using a 168 grn SMK moves at about 2730fps and creates pressures in the 50,600CUP range)
This data is available here:
www.flintwood.co.uk/reloading/308win.htm and I have referred to this site (which has a link to Hodgdons) and his loads on several occasions for figuring out where to start and what my limits are. That is when my current gunsmith and friend does have a load already defined and worked out for what I'm wanting to shoot.
Be safe, start low and work up, yada yada yada....
Mike