Grendel to ARC Brass

[BassinDmax]

I'm still a couple months out on my barrel, but I've been trying to purchase all the reloading components I'll need. I didn't want to keep holding out for ARC brass so I picked up 300 pieces of Lapua 6.5 Grendel brass. With there being some differences in the shoulder from Grendel to ARC, my question is, when I size this brass to 6MM ARC how far do I push the shoulder back?

Is this for an AR15 or a bolt gun, or bolt upper for AR15 lower? My prep would be different for each.
I am waiting for bolt upper for AR and will be shooting with both. Most in this thread I would apply to a typical bolt gun head spacing, ultimately using fired cases for ongoing reloading. Without barrel in hand, what to do? 
I will use lapua cases for 6.5 grendel when available for bolt upper(Uintah). I will likely use Hornady cases for the AR. Currently, I only have about 20 fired cases(Hornady). I will consider some of these sacrificial cases. I purchased a Wilson “case checker” which can assess whether or not a case is within SAMMI specs.

I slightly undersized a fired case in a RCBS small base sizing die with the expander ball in place.
I put the case in the Wilson gauge and turned in the die and resized until the headspace part of the die showed the case was within spec. I then trimmed the case neck until it was even with the minimal SAMMI part of the gauge.

This gives me a case that should fit even a tight chamber due to small base die. I will fire in AR15 barrel. When I get my Hornady dies, I will do the same procedure for initial firing in the bolt gun, using the small base die only if necessary. I will get a custom die for the bolt gun when I have fired cases to send.

I will likely use commercial die for the AR. More generous setback to AR shoulder vs 2 thou or so for bolt gun.JMO

This is a gas gun, I'll be running a 20" X Caliber barrel. I like you method plan, I think this will work in my situation also.