A first approximation divides available cash by the net monthly burn. It works best when the pace of receipts and payments is steady.
An example with numbers
With $30,000 available, monthly payments of $12,000 and receipts of $7,000, the net burn is $5,000 and the simple runway is six months. If a one-off $10,000 payment is also due in the first month, it drops to four months from today under the other assumptions.
Try it with your numbers
Place the one-off expense in the week it is actually due. Check whether cash runs short sooner than the monthly average suggests.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.