I am playing Pokemon GO and have caught an Eevee with 376 CP. How high will its CP be after evolving?
This is a difficult question to answer without knowing how the server generates Pokemon evolutions but I'll try and push you to the answer you need.
Behind your Pokemon you will see a semicircle with a dot on it, this indicated what percentage your Pokemon is to achieving it's maximum cp (This number changes every profile level-up). your Eevee might for example have a max cp of 400 so your percentage would be around 93.75% of your Pokemon's maximum. Now when you evolve a Pokemon what happens is the new Pokemon gets that percentage applied to it (93.75%) but since it will now have a higher maximum cp, for example 1000, It's new cp will be 93.75% of 1000.
Now taking this into account, we don't actually know yet how the Pokemon's max cp scales with accounts, we can only guess, but it will be scaled to your pre-evolution Pokemon