A new suggestion

I think you could just do that with the CP calculator:

https://pokemongo.gamepress.gg/cpcalc#/

Yeah just put in your Ditto's IV and lvl but use the pokemon it's transforming to

I live in a town with 7 gyms in a 1 mile radius. I take all 7 every morning and lose them by 7 PM ever day.
I see dittos and they just are not good defenders at all.
My 2835 Rhydon can easily clear 3 of them and not require a revive. Because I take every gym every day, they rarely get higher than level 4.