Ok, this is weird. I am sure that in the past I've seen war exhaustion increase at a constant rate for all war participants; evidently, I was wrong.
I just did a quick test in vanilla to double-check my calculations, and here are the results over a 12-month war with Magna Graecia starting 1.1.474:
Month 1: ROM 0.00, MAG 0.04 (??)
Month 2: ROM 0.08, MAG 0.72 (at this point I had just defeated their 6k strong army)
Month 3: ROM 0.08, MAG 1.29 (no idea what this jump was about)
Month 4: ROM 0.10, MAG 1.29 (no change?)
Month 5: ROM 0.11, MAG 1.44 (what? I destroyed a 1000 man army, was that the cause?)
Month 6: ROM 0.13, MAG 1.48 (by this point I had taken Tarentum and had started a long siege of Ager Bruttius, so the increases are a lot more steady)
Month 7: ROM 0.15, MAG 1.52
Month 8: ROM 0.16, MAG 1.56
Month 9: ROM 0.18, MAG 1.60
Month 10: ROM 0.19, MAG 1.64
Month 11: ROM 0.21, MAG 1.68
Month 12: ROM 0.23, MAG 1.72
Sigh, take back the second half of my last post, it's clear I don't yet understand how war exhaustion works
