Projections for scaling can be difficult.

For Facebook ads, part of the question will be whether you haven't captured all of your core audience because of budget. If you have narrowed down what's working and have been hamstrung by the spend, then Facebook projections can be pretty reliable as long as you're maintaining the core demographics.

For AdWords and paid search, if you're losing impression share due to budget then projecting based on that will be pretty reasonable. If you have to expand into new keywords to make the budget expansion happen, I would be conservative with the estimates and would not assume a 1 to 1 based on past performance.

Hope this helps!
best,
Paolo