What you're doing with shoulder setback is changing (usually raising) the depth of your resizing die in the rock chucker (or whatever your model is). Most resizing press mfrs say to set the die so that it just hits the seating collet on a full-up position. That can set a shoulder back to "Saami" but probably sets it too far back for a given guy's rifle. You want the fired shoulder to tell you where your chamber leaves that shoulder after firing, so you can tweak it back a few thou's from there, so it works for the next round of loads, but also stretches and sizes the brass the smallest amount for best brass life.
You can start by putting in your fired case (you already had measured its shoulder) into the resizing press and resize down to whatever the mfr setting will give you (by setting the die to seat flush against the holding collet). Then measure the shoulder setback. For example the fired case might be 1.193 and resizing brings it to 1.180 - a 0.013 delta. Lots of brass-working going on there! You want less than that but still Saami and workable in your gun. Let's say you want close to 1.185 final (delta of 0.005 is where I go and lets you account for some springback and still be ok).
So the resizing die and the press usually have a thread pitch like 7/8-14, the 14 is what you need. Math-wise a 14 pitch means 1 turn moves the die 1/14th of an inch, or 0.071428"... 0.003 can be found then by (just follow my math here) 10deg= 10*0.071428/360 or 0.00198. So if you back off from the mfr setting by 10 deg, you have moved it basically 0.002". 15deg = essentially 0.003. So take a sharpie and make a match-mark set on the locking ring+die body at a convenient point, then make a 30 or 90deg mark on the ring, then cut it down to 45 or 30 to get to ~15 deg. 30 deg is easy to mark and would leave setback at 1.187 (~0.006 from fired). The die body will move ccw relative to the locking ring so your 30deg mark on the locking ring should be in that direction, then you can loosen the ring and move the die body out, matching up your "base" mark on the body to the "30 deg" mark on the ring. Measure another fired, then resize with this new setting to see where you are shoulder-wise. Tweak and adjust from there. Do a sharpie final match-mark set to document where your setting is.
This gives you a logic and a method for marking so you can get to a setback you want, plus if you ever loosen the locking ring you can get back there with your final match-mark set.
Sportsmans.com and a couple other places have a hornady LNL modified case, 6-9 bucks plus whatever shipping... throw a couple other things in there too if you want...
You can start by putting in your fired case (you already had measured its shoulder) into the resizing press and resize down to whatever the mfr setting will give you (by setting the die to seat flush against the holding collet). Then measure the shoulder setback. For example the fired case might be 1.193 and resizing brings it to 1.180 - a 0.013 delta. Lots of brass-working going on there! You want less than that but still Saami and workable in your gun. Let's say you want close to 1.185 final (delta of 0.005 is where I go and lets you account for some springback and still be ok).
So the resizing die and the press usually have a thread pitch like 7/8-14, the 14 is what you need. Math-wise a 14 pitch means 1 turn moves the die 1/14th of an inch, or 0.071428"... 0.003 can be found then by (just follow my math here) 10deg= 10*0.071428/360 or 0.00198. So if you back off from the mfr setting by 10 deg, you have moved it basically 0.002". 15deg = essentially 0.003. So take a sharpie and make a match-mark set on the locking ring+die body at a convenient point, then make a 30 or 90deg mark on the ring, then cut it down to 45 or 30 to get to ~15 deg. 30 deg is easy to mark and would leave setback at 1.187 (~0.006 from fired). The die body will move ccw relative to the locking ring so your 30deg mark on the locking ring should be in that direction, then you can loosen the ring and move the die body out, matching up your "base" mark on the body to the "30 deg" mark on the ring. Measure another fired, then resize with this new setting to see where you are shoulder-wise. Tweak and adjust from there. Do a sharpie final match-mark set to document where your setting is.
This gives you a logic and a method for marking so you can get to a setback you want, plus if you ever loosen the locking ring you can get back there with your final match-mark set.
Sportsmans.com and a couple other places have a hornady LNL modified case, 6-9 bucks plus whatever shipping... throw a couple other things in there too if you want...