You're adding absolute dollars rather than using percentages - that usually isn't how that works.
Not really.
Fractions (7/2), ratios (3.5x) and percentages (+250%) are fundamentally mathematically identical.
There are a lot of problems with this back-of-the-envelope estimate, but I’m not sure the one I understand you presenting is one of them.
Of course you don’t use percentages when the magnitude of the numbers are so high.