I wish I could help you specifically with That cartridge. Its a fairly new cartridge.
Here is load Data for a 28 Nosler with retumbo & a 175 gr partition with retumbo. It's my understanding that capacity of both cartridges are roughly 98 grains h2o. There is .007 difference in bore size between the two. My 7mm rem mag has a capacity of around 82 grains h2o. 71 grains retumbo is near max in my 7mm rem mag with a 162 gr and 70 is near max with a 180 gr in my 7mm rem mag. This chart says 72.3gr retumbo as starting load for a 28 Nosler with a 175 gr. If I were loading a 170 in a 27 Nosler, I would start with around 73 grains of Retumbo and carefully work my way up. Use a magnum primer.
Comparing the 27 and 28 things get kind of weird despite a narrow barrel size difference and similar case capacities. If you look at the IMR 8133 loads for 150 grain bullets for both (published here
) -- the 27 has a min/max of 73.5/77.5 while the 28 has a min/max of 78.7/85. Going to 30 Nosler you get min/max at 83/91C for the 150's.
For 175 grain loads of Retumbo, the Hodgdon's site has the min/max at 72.3/78. That's in line with what you have. The 30 Nosler does a 180 grain with Retumbo at 76/83.
If all of this works out, it looks like I should try running 73-ish grains as the min (like you suggested) and work up +.2 increments until I see pressure.