I use the RCBS 1630 Neck Die, but if you want to use your FL die to work similar in function it's very easy to do. But it won't provide you with true neck sized brass though. And the necks will very likely only get partially resized, which isn't usually an issue as long as you are getting enough neck tension.
First thread the die into the press with the ram at full extension. Adjust the die down to where it is roughly .010"-.015" from the shell holder, that is plenty to avoid shoulder contact. If that touches the shoulders, then that chamber is excessively out of spec.. Then adjust the de-capping pin down another 1/16" or so and make sure it is centered, it should be if it hasn't been previously bent.
Check your cartridge with a gauge prior to, and afterward to make sure the shoulders didn't get bumped, just to be sure. If the first one got bumped at all then continue adjusting the die up another .010"-.015", but I've never seen a chamber that sloppy before.
But, because you are using a FL die the case body is still going to get sized some. And because of this it can push the shoulders up some, this is due to the brass being displaced, it has to go some where.
Don't forget to trim follow all of this. PM me if you have any other questions or concerns.
GS