Ideal CP to attack GYMs?

I know it used to be that you got high prestige gain from attacking with mons less than half of the cp of the defender, so try to build trams around there.

Keep in mind that you get max prestige if your strongest pokemon has half of the defending pokemon's cp or less

in Paris most gyms start at 2300 or 2500 CP, that means you have to use 1000-1200
in my rather rural city, it usually starts at 2000 more or less. but sometimes the first defending pokemon's cp goes down to 1000-1500

so i try to have good pokemons with good moves but low cp, basically anything under 1500
sometimes i have to use a team of 6 pokemon under 250 pc, it's ridiculous :D

I usually don't go for Max 1-1 Prestige instead I plan on beating 2-4 Pokemon.

I find 1150-1350 CP my sweet spot for attacking Gyms around 1800.

Then I pick 2 pokemon with type advantage over first defender.

Then I pick 2 pokemon with type advantage over next defender.

Then often theres a 3rd defender that shares a type, and sometmes a 4th with defender 1 or 2. My 5th pick is this type.

My last spot is clean up. usually a wigglytuff.

Then I beat 2-4 Defenders while I'm not getting 1000 (500 under new rules) for first guy, as a total I spend less time picking a team and less potions since my max isn't as far down.

half of the defender cp, thats how i play

I try to use around 1100-1300 cp for gyms with Pokemon over 2k. Not sure why but it seems to me like the 1000 CP pokemon and below are much weaker more than proportional to the 20% difference in size. Plus sometimes you miss a dodge and lose and that wastes alotta time when you need to train a gym 40-50 times.

No point training friendly gyms anymore after prestige change. So no need for any of those small guys anymore. :p