The arithmetic
Take a specialist’s contracted working hours across a full calendar year. Subtract Thai public holidays. Subtract their annual leave entitlement. Subtract an allowance for personal leave. Divide what remains by twelve.
That figure is the committed monthly capacity written into your agreement, and the price attached to it. For a typical full-time specialist it lands around 160 hours a month.
Why some months run over and some run under
A month with no holidays will deliver more than the committed average. A month containing Songkran will deliver less. Both are already priced in, because the commitment is an average rather than a monthly target.
Your invoice is identical in both. So is your forecast.
What the fixed fee does and does not do
The fee is a committed monthly capacity rather than a measure of hours consumed, so it does not move because one month had fewer working days than the next. That is the practical difference between a managed agreement and an hourly contractor: nobody on your side audits a timesheet.
What a fixed fee does not do is make an unavailable specialist available. If someone is off for an extended period, that is a conversation about the plan, and it is worth agreeing up front with any provider how that conversation goes rather than discovering it in month seven.
The same question is worth asking of staff augmentation, where the arrangement supplies a person rather than an outcome and the schedule is yours to manage either way.
Raising or lowering the commitment
Capacity is reviewed at the monthly cycle. You can add hours, add a discipline, or reduce, and the new figure applies from the next cycle. Nothing is renegotiated from scratch.