I’d back ‘em up to 12 miles from the border. No point in risking a stray round landing in a foreign country...
Paladins are pretty damn accurate. (I was in a FA unit with them.) and you could also have a quarter-mile "buffer"/grace-zone.
The blast-radius kill-zone of a 155mm HE round is 50m (≈164.042ft) 1/8mi is 660ft… so 660ft - 165ft = 495ft — plenty of space, and that's with a mere 1/8mi buffer.