Enough to meet fixed obligations through the trough the model identifies, with margin for a season that underperforms. Work from the actual commitments — lease, payroll, insurance, loan instalments, minimum inventory — rather than a percentage rule. Capital held for that purpose inside the enterprise supports the case; the same money left in a personal account does not.
Price the quiet months as a line item
List every obligation that continues when sales stop, total it across the slow period, and fund that total deliberately instead of hoping the peak stretches. Keep this figure separate from professional fees, government charges and relocation costs, and confirm current official amounts at the time each step is taken. If the reserve requirement changes what the investor can commit to the purchase, that is a reason to revisit the deal, not to shrink the reserve.
Hypothetical example: an investor considering a school photography business finds that almost all revenue arrives in two autumn months while equipment leases, storage and a small year-round staff run continuously. A first review would total every obligation that continues when sales stop, multiply it across the quiet period, and treat that figure as capital the enterprise needs rather than as a contingency. It would also ask whether that reserve, once funded, leaves enough to complete the purchase at all.
If it does not, the answer is to revisit the price or the deal, because a reserve trimmed to make the arithmetic work is the first thing a slow season removes.