As a rough rule, most companies pay their reps as some % of the first year total deal value.
For annual deals, this is quite easy to calculate.
For monthly deals that are non-variable, most folks just assume an annualized value, and either do a claw-back if the account churns in the first year, or do a lower % of the value vs. an annual deal.
For variable deals like this, there’s no perfect formula.
For now at least, I’d make an educated guess (and combine it with clawbacks). And pay a fair commission as close to up front as you can.