If I have two of the same Pokemon but with different CP but the same progress on their CP bars and I decide to evolve both, do they both receive the same CP boost directly from the evolution or does it vary by it's original CP?
Yes. See the circular Combat Power bar behind your Pokemon's model? It stays the same percentage filled on evolution.
The max CP of a Pokemon depends on weight and height I think. In your example of the Rattata's with the 50% CP bar and a difference of 10 in CP, one Rattata is probably bigger.
If you evolve or power up the Pokemon it seems to go up with an x-percentage of the max CP of that level Pokemon. (Based on the fact that costs for powering up are same before and after evolution)
So if you evolve both, their CP level will probably show the same increase in percentages. I don't think it will be easy to figure out the exact formula. Since height and weigth seem to determine the max CP level. Not sure about this theory though, but that's what I make out of it after reading a lot and seen examples.
If your CP bar is full and maxed out for your Rattata, it will also be full and maxed out on your Raticate after evolving.
So basically if you want to reach the highest CP possible for a certain kind of Pokemon, you need to find the fattest one out there. And then you can evolve and power it up to the max
Phoenix is totally wrong. CP is all over the map and isn't determined by weight or height or anything logical. Still a mystery.
I currently have 4 Pidgey's I'm experimenting with. The lightest and shortest Pidgey is the second highest CP. Also the highest CP Pidgey isn't the highest Hitpoints, which is important for battling. Sure the highest CP Pidgey is the heaviest and the tallest, but CP isn't all that goes into battling. They have to be able to take the hits too so Hitpoints are important. None of these Pidgey's have been powered up at all. Given these 5 Pidgey's, I feel like the one to hold onto, without factoring in attack moves which are important too, would be the Pidgey with 191cp or 174cp. I could power each of these up to 195cp and they would each have a lot more Hitpoints then the current 195cp Pidgey.