The standard "Go-To" load (or at least mine) is 25.0grains of H335 with the 55g bullet. Shoots well in almost all rifles, and where I start with any .223. Rarely is anything else necessary to try. Less than a maximum load in all books. (You picked the right powder right off the bat!)
For this bullet, my speer manual #12 has a starting load of 24.0 gr h335 with a MV of 2805 fps. max load is 26.0 gr at 3092 fps. C.O.L. tested was 2.175". A CCI 450 primer was used, IMI brass, test firearm was a 22" bbl Ruger 77 Mk2 with a 1-12" twist. hope this helps, Mike.