That's my chambering and bullet, I use Varget (for all my 223/5.56 loads). I got my min/max load data from the
Hodgdon site. It says 24.6gr max with the 75.
I chose 24.4gr as a personal max load with Varget and fired it in comp at 600yd. It worked, and the (new Winchester) brass came out without ejector swipes, the primers were flat/cratered, but not greatly so. I used the load in a 24", 1:9" Savage and a 24", 1:8" Stag, and they held the 9 ring at 600yd (1500ft altitude) with two different shooters, myself and my Granddaughter.
For several years (maybe getting up near ten years), I was using the HDY 75gr HPBT-Match at 300yd and 800ft altitude in both of the above guns, and used a reduced load (23.7gr Varget) with excellent accuracy and moderate pressure (the brass/primers show a lot less pressure indication).
These days, living and shooting at 4200ft Altitude, the 23.7gr Varget load (in new Starline brass) has returned to being my goto load. For all but actual match shooting, I use the CCI 400 primer, and the BR-4 for competition. The difference is so subtle, I'm not really sure the BR-4's are doing anything but costing me more.
Unless you're shooting max distances - 600yd and beyond - at much lower altitudes, the near max load is not really needed. Start your load testing in the 23.5gr-23.7gr range with Varget. I doubt you'll end up more then .2gr out of that range, if at all. Be aware that the 75 is far from immune to wind at 300yd, but if you can manage the wind, it's stellar.
I don't know much about the Ranch rifle, and have reservations about the 75 in any 1:9" twist with significantly shorter barrels; I would be more comfortable using a 1:8" or even 1:7" twist in shorties. If you find it keyholing, step down to the 65-69gr bullet range. The Sierra 65gr Gameking is a very accurate bullet with good terminal performance on medium size game.
Greg