The money to open must also cover the gap between payments and receipts. A refundable deposit, for example, ties up cash even though it is not a permanent loss.
An example with numbers
You pay $6,000 for equipment, a $2,000 deposit and $1,500 a month in cash expenses. If you expect two months without receipts, you need $11,000 for those items. Another month of delay raises the amount to $12,500.
Try it with your numbers
Build a monthly calendar with investment, deposits, operating payments and first receipts. Look for the largest cumulative shortfall.
This calculator is on its way. In the meantime, you can do the calculation on paper or in a spreadsheet.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.