Sup guys...

So I have a CP430 eevee waiting to be evolved.... Would it be better to power up first and then evolve, or just evolve and power up after?

I would assume it grows exponentially.

I'd say level then evolve, since when evolving their cp is roughly doubled

my 410cp eevee went to 1087cp vaporeon thats way more than double

Sometimes its less than double, but in the end its roughly doubled.

This is correct.

I read somewhere that the pokemon are on the same scale and we just find different versions of the same scale. The theory behind it is that it doesn't matter when you level up, a fully leveled pokemon will always be the same end result.