IVs and Level

It depends on your goals. If you want to be the best, like no one ever was, you’ll want higher IV Pokémon that you’ll invest stardust and candy into to power up. If you want gym fodder, anything will do.

I think many players keep high CP/meh IV Pokémon until they get better alternatives.

Also, only put resources into Pokémon you know you’ll use. Whether that’s for taking gyms (Machamp, etc) or current raid bosses (Tyranitar, Mewtwo, etc) or just something you like, make sure it’s a decent investment.

IVs’ importance is somewhat of a debate. If you look at the math, slightly lower IV Pokémon can perform almost identically to 100% Pokémon provided they reach the same breakpoints. There’s guides on this site that go more in-depth and explain how breakpoints, bulkpoints, and IVs work.

A good rule of thumb is generally the attack IV is the most important one (though this may change with the recent announcement)

You'll get more immediate use out of a level 30 78 IV (12-13 atk) but you'd invest the resources into the level 18 98 IV (15 atk) all else being equal.

That said, the resource savings on a lucky are significant and with some moves (Smack Down Ttar or Shadow Claw Gengar), any IV is worth improving.

Have a read at this guide about breakpoints that explains the relation between level and IV:

https://pokemongo.gamepress.gg/guide-breakpoints

Before TMs were a thing, species was most important, then move set, then IV. The reason being that the IV only contribute a few percent of a mon's performance. When you first start powering up a mon, the gains in performance from powering up, are greater than the performance gain of having high IV. It is especially in that last expensive stretch between lvl 30 and lvl 40 that the benefits of powering up further are outweighed by the benefits of having higher IV to start with. So, level matters at early levels, and IV matters when you want to invest heavily to squeeze the last few percent of performance out of a mon. Or, at least, that's a good, approximate way to think about it.

IMO people put too much weight into IV. A 10/10/10 Raikou with Wild Charge is still 60 sec ahead of a perfect Zapdos with Thunder Shock against Kyogre at level 40.

Another example: A 10/10/10 level 40 Mewtwo with Shadow Ball is only 20 sec behind a perfect Mewtwo (against Mewtwo), and still 30 sec ahead of a perfect Tyranitar.

In your example, a level 11-12 perfect pokemon would be equivalent to the level 10 you described.

My point is, don't be afraid to power up a 89% legendary if you didn't find a better one. Usually you also loose significantly less time as long as attack is high enough (breakpoints).

you have to do some calculations to find the exact value of IVs compared to level and it depends on the species. It's more important for weaker pokemon and less important for strong pokemon

It's easy to just compare the stats.
Attack = (Base Attack + Individual Attack) * CP_Multiplier
Defense = (Base Defense + Individual Defense) * CP_Multiplier
HP = (Base Stamina + Individual Stamina) * CP_Multiplier
CP multiplier values can be found here https://pokemongo.gamepress.gg/cp-multiplier

At level 40 the multiplier is 0.8 so for a maxed pokemon the perfect IVs add 15*0.8 = 12 to all stats. So dragonite's attack will go 263 > 275, def 201 > 213 and stamina 182 > 194
So stat-wise perfect IVs compared to 0 IVs is 5.5%. In reality there's breakpoints so it could be slightly more or less impactful (usually less)

Fun fact: A perfect rattata will still get +12 from IVs. Which is a lot compared to rattata stats. So for rattata perfect IVs make his stats 15% stronger! If you want to fight with a rattata, definitely search for a perfect one! :P

TL;DR: perfect dragonite lvl 27 is the same as 0 IV dragonite level 30. Perfect 22.5 level rattata is the same as 0 IV rattata level 30.
IVs only matter for similar level pokemon to hit breakpoints and usually it's just the attack IV, so that your dragon tail will do 13 damage instead of 12.