I know this is a bit old, Zen Fox, but I've been reading through your threads on mass combat to try and get a grip on the system for my own campaign. This one gave me pause, and I may be off - but it looks like you are calculating the Spellcasters bonus based on the caster level, and not the spell level. So an army with 4th level casters would be granting a +2 bonus to their army, whereas a 17th level army would be contributing +9 - the maximum.
I am referencing the SRD rules (Ultimate Campaign) for these. Not sure if the other systems used a different metric for Spellcasters.