T. O'Heir wrote, "The 145 and 160 grain data on Alliant's site(very decidedly limited. They want you to buy their book. Max loads only there too. Reduce by 10% for the Start load.). Either will do for a 150. I'd lean towards the 145 data though."
Actually, I'd lean to the 160 data as charge weights decrease with increasing weight of bullet. Instead of leaning, it's better to use a mathematical process called interpolation. Let's form a linear expression in the weight of bullet for the charge of powder:
c(w) = (c1(w2-w) + c2(w-w1))/(w2-w1)
So, c1 is the charge under the first bullet and c2 is the charge under the second bullet. w1 is the weight of the first bullet and w2 is the weight of the second bullet. When w is equal to w2, the expression comes to c2(w2-w1)/(w2-w1) = c2 because the other term has a factor of zero. So, with w=w1, the expression becomes c1, just what we want. If this seems odd to you, ask a 15 year old maths student.
Plugging in w=150, we get c(150) = (67(160-150) + 64(150-145))/(160-145)
c(150) = (67X10 +64X5)/15 = (670 + 320)/15 = 990/15 = 66 grains of RL-19.
Doing it mathematically doesn't make it right. One still has to use the same brass and LOA of Alliant and hope the chamber and throat are similar so starting 3-10% lower and working up in steps is wise. Starting lower compensates for the possibility of quite different cases, chambers and throats of rifles that are out there. Don't overdo it though. Slow powders usually don't work well in very low charges. I usually find 10% is much too conservative and some slow powders get really dirty or give ragged velocities. I have found 3% is not quite conservative enough as some bullet-jackets and chambers are quite different. I've seen one rifle that would not seat a bullet out as far as SAAMI specs. I've seen variations in lots of primers and powders exceed 1%. I've seen powders whose maker recommends an exact recipe with no deviation. Hate that. It takes experimentation to find what works well with what you have. I tend to use 5% as a compromise.