you may be able to program in a work counter that uses an IF/GOTO to jump it to a sequence to make the bar loader push after the 12th part. THen the loader will hit the end of bar position and automatically go in to a new bar load sequence. for instance
(End of Part 12)
iIF V1=12 GOTO666
N666
(push bar)
Or how most people run loaders is they use the loader to push the bar to a bump stop every new part and when it hit the end of bar position it automatically lads a new bar. This way it doesn't matter if you have the same length bars every time either.