JCL : How to calculate REGION size

Look at a recent run of your job. In the JESYSMSG you'll see a line similar to this:

VIRT 6408K SYS 548K EXT 6328K SYS 10868K

The first two number represent the below the line region, the other two represent the above the line. Add them together and that's the region required. In this example I'd use REGION=24M.