Two questions about my gengar

If you're on Android, just go download "IVStats"

Seriously, don't even finish reading the posts. Just go download it. You don't give it access to your account, but rather post a Google authentication key. It's perfectly safe.

It has saved me SO MUCH time and effort.

Not only does it give you IV's, but it has a power up cost calculator (how much candy/startdust to take to level XX), it has the CP after evolution, it has the max possible CP at level 30/40, AND it has links to gamepress's (this website's) moveset rankings. PLUS it has a built in Pidgey calculator.

it's really the best thing ever

There are alternatives for iPhone, but I'm just not aware of which are the best :)

Hi. I have a Gengar lvl 28 with 87% IVs and the same moveset. I have never used him in battle, sadly. If he didn't have poison type, he'd be useful for Psychics (read:Exeggutor), but with that weakness he's just too fragile, and other attackers in my team are more fit for the job. It's sad, because he's a really cool mon :)

Gengar has always been my favorite Pokémon, and in the main series he was very effective as a versatile wall breaker, glass cannon type due to his amazing SP ATK and speed. But in pogo he's basically useless ...

If he had a better fast attack than shadow Claw he might be more useful, but SC is a bit too clunky to spam and dodge enough attacks to be effective similar to Alakazam. Sadly, until Gengar gets a better fast move, he's not very useful.

I've got a 93% IV shadow claw - sludge bomb Gengar and I've given up spending dust on him for now.

Like some people have already said: Gengar isn't the best pokemon in pokemonGo. But I have used a 2227 Gengar to take down some lower level Laprasses (around 2.2k) quite effectively before. I don't use him alot but he's to use every once and a while, and that's more important to me then saving a bit of time using the most 'efficient' pokemon.

However: if you don't have alot of stardust and have some other good pokemon that you can power up: power those ones up first.

The only way that I am aware of to determine which IV is correct is to level it up. At some point the CP should get to a value where the calculator can tell the difference between them (although i'm not sure this is true in 100% of the cases). Which I realize defeats the purpose of you other question about whether to level it or not. In either case I think the IV's are close enough that in actual combat performance it there would be a negligible difference between them.