Unfortunately, I don’t have a rule of thumb. Figuring out the amount of below-the-line DSA has always been a tricky balancing act between CICS and operating system requirements, which is usually established by trial and error.
I would not increase DSALIM unless you think there will be enough storage left over for the operating system. If there isn’t, you’ll put yourself in the position of getting 80A and 40D ABENDs.
You can establish if you have any room to increase DSALIM by watching the available storage between the top of DSA and the bottom of system storage. If you leave your CICS regions up for days, you’ll have to monitor that area for fragmentation. If, after your monitoring, you think you have some cushion you can increase your DSA to take up the slack. Otherwise you may have to follow the classic CICS solution of growing horizontally.
Also, do not specify REGION=0M on the CICS job step. While coding the REGION parameter in this way assures the application has access to every byte of private storage below the line, it does not leave anything for the operating system. Again, you will be vulnerable to 80As and 40Ds.
By the way, increasing the DSA below the line shouldn’t impact EDSA above it. The two are separated by system control blocks, the MVS nucleus and Link Pack Area (LPA) modules.