How can you forecast the % increase in CPI in relation to % increase of daily spend on Facebook?

One way is to look at your current budget as a ratio of the potential audience for your default targeting. Then take a segment of that, which you know/think performs similar to your broader targeting, and spend at the budget/potential audience ratio you're trying to forecast. The CPI you see there can be a directionally useful number for your forecast.

I think the only logical way to understand the ratio is to test it with your own data. With this I mean, increasing the budget by a small percentage (10%-20%) in every 2-3 days and observe the change on your CPI's and repeat that for up to 10 times until you have enough data to come up with a correlation.