This should be easy in an established business. Create a weekly cash flow model for the last couple of years using the actual numbers from the business. Assume some reasonable amount for starting cash. Each week is one column on the spreadsheet. Sales, taxes and expenses are the rows. Enter the income and the expenses (In this example you must include sales taxes both in sale and when the payment to the taxing authority is due)
Starting cash+
Sales
-Expenses Paid
Rent
COGS (actual $$)
insurance
Advertising
Wages paid
Etc Etc Etc
=Ending cash
Ending cash becomes the starting cash for the next week.
Expenses come from actual checks written as of the date of check, CC payments as of date when you paid the card, and cash payments from the POS as of the date spent.
Your spreadsheet will show you when your cash goes negative. In most businesses this is reasonably predictable.
The amount you need is enough to cover the biggest shortfall plus some amount for unforeseen or irregular costs. I would budget irregular costs weekly. For example, if your repairs budget is $2,600 per year I would put it in as $50 per week rather than whatever random dates the money was spent in the last year. If you are worried about being closed for some period of time I suggest business interruption coverage rather than holding cash just in case.
You do not need to have all of this in cash. If you have a line of credit you can draw on it and replace. For example, in my business we always went negative on cash flow in the mud-season. I could either hold enough cash to cover it or draw down the credit line and pay it back. As an example, if you needed to draw 10K to cover a 6 week shortfall and your interest rate was 6% it would cost you about $70 if you waited until the end to pay it all back. Less if you paid it back as you went. In slower seasons I often would draw down my credit line on a Thursday for payroll and pay it back by Tuesday when the weekend business posted or pay down my line all month and then draw it back for the CC payment… and then pay it down again.