I work for a digital advertising agency. We fundraising for our clients and often get asked how much more money can we spend and achieve the same net profit even if the ROAS (return on ad spend) is lower and what is the degree in which we can increase spend with a specific ROAS that would actually bring in a higher net profit.

I need a formula, ideally something I can input into Excel, for calcutating this on the fly. I want to say it should be a profit maximization formula, but that doesn't quite capture it.

For example, if I'm spending $1000/day and raising $2000/day, with a net profit of $1000/day and 200% ROAS then what is the minimum I need to spend and at what percentage of ROAS (assuming it will be lower as spend increases) to reach the $1001/day in profit