I would just start around 0.010 off and find your powder node first. I do this from starting about 2-3gr under book max or from what others are using. Then load one round each in 0.2gr increments from the start to max. Chronograph every shot and write the velocities down in the order you shot. Also while doing this, watch for pressure signs like heavy bolt lift and ejector marks. I have learned that certain primers and loads will flatten primers even at mild charges, so I rely less on what the primer is showing me and more on bolt lift and the ejector marks.
After shooting these rounds, there will be a clear spot in velocity where a 2-3 charges produce very close velocities. That's the node you're looking for. If you aren't happy with the speed you're getting, and you never maxed out on pressure, then I would slowly work up from your previous highest load in 0.1-0.2gr increments looking for the next higher node. After finding my velocity node, I'll load up 5 rounds in the middle of the node and test them for consistency. Normally, doing this will produce a 5 shot average with extreme spread 20 or less and single digit standard deviations. You must have a good scale and a good technique in order to make accurate and consistent loads.
My final step is shooting groups to fine tune my seating depth. If 0.010 doesn't shoot, then I start working back. I'll load 3 each in 0.010 increments from 0.010 to 0.060 off. This will narrow down your seat length "window". You should find a seat length from those groups that shoots better than others. Then from there, you can load 3 rounds each 0.002 deeper or longer until you find accuracy.
For a 7mm Rem Mag with 180s, I would recommend H1000, RL26, IMR 7977, and Retumbo. Find one of those powder you can get a surplus of and roll with it. I recommend Federal 215s, either standard or Gold Metal Match, or Remington 9 1/2 Magnums, we have had the best luck with those. And for brass I would get ADG or Norma.