What level do you stop at to get a featured character to 166 with the level boost?
Featured characters seem to have their level multiplied by 1.5, so 112ish should be the place.
See NP’s Guide:
https://forums.505go.com/discussion/3939
Basically, get a featured to 111-112 if you want your featured PVP to tank (eg boost to 166+), and 110 if you don’t want them to tank (boost to lvl 165).
Since the featured character is always in the middle, they will always tank on color ties, so boosting from 111 to 166 is actually good enough I believe.
110 would be 165 with 50% level boost
111 would be 166.5 with 50% level boost, and I don’t remember off hand if the levels are rounded or truncated, but either way, it’d be either 166 or 167 so it’s still the closest you’ll get to 166, unless there’s reason you don’t want to go above 166, though if you don’t want to go above 166 I’d imagine you don’t want that character to tank and in that case being 166 that character will get TU and any other ties on tiebreaker since the featured guy is always in the middle.
I appreciate the responses guys 
They round up on the odd levels when boosted, so you can either do 110/165 or 111/167. I know this because I always put ISO into the featured character, and 111 is one of the milestones I try to hit. The other is 133 so they boost to 200, which can scare off some people.
Yea I’ve been meaning to incorporate this strategy for a while now but 4*'s are just devouring my ISO so it doesn’t leave much juice for the lost boys on the back pages lol
I left my XF at 221 so that I could have more ISO for my three stars. Once I get a usable 4Thor (currently 5/1/2), I will probably level them both up to max, but having a non-useless featured can help me take down all those max 4* teams until I have my own.
I’ve been running both X-Force and Goddess at 220 since just after she was released and it’s been fine. I’m in no rush to0 level max them.