Start real low, like 35-36grain then load one bullet in each 0.5grain increment till you notice a sticky bolt, flattened primer, on an ejector mark (shouldnt see this unless you got wayyy too much pressure). Once you notice any of these signs back off atleast one full grain, that's your max for that batch of powder, with that primer that bullet at that dept, using that batch of cases. If you change any of those variables, back off a bit and test again. May seem like a hassle but beats having your gun ruined, your face reorganized, or having loaded an entire batch of ammo that you can't shoot and have to pull.
And Buffalobob is right, the sierra mks have a thicker jacket than your average bullet and that can cause more pressure. There is a reason reloading manuals tell you never to start at or near the max, this is it.
Stay safe,
Oliver