Set the hours you’re reserving each month and what you charge. This gives you the monthly figure to quote, what it’s worth over a year, and the rate you should bill anything past the cap.
$20,196 a year if it runs twelve months
The buffer is applied after the discount, so the discount lands on the work itself rather than on your operational overhead.
The client isn’t buying hours. They’re buying the guarantee that you’ll have room for them, which means turning down other work to hold that room. The discount is payment for prepayment and predictability — not a volume deal.
Let them expire at the end of the cycle. If you allow hours to bank, a client can sit quiet for four months and then ask for eighty hours in one, which wrecks every other commitment you have. If you need to give ground, cap rollover at 20% and expire it after thirty days.
Invoice on the 1st, before the work starts. A retainer billed at the end of the month is just hourly work with extra paperwork, and it puts you back in the position of chasing money for time you’ve already spent.
Get written approval first, then bill at the full non-discounted rate or higher. If overage costs the same as retained time, the client has no reason to size the retainer honestly.
Estimate the hours each deliverable takes, run the total through the same calculation, then quote the package rather than the hours. You still want the internal hour figure — it’s how you find out three months in whether the package is quietly losing money.