For those who caught a few close IV mons and don't know which one to power up

I have met this problem recently where i have caught a few legendary Pokémons and planned to power up one of each. But a couple of those have really close IV and with different ways of measuring them, i can power up either one of them. So now, i want to talk about a couple of ways I thought would help me to decide.
1. The easiest one to use: check the IV from any IV checker sites. All three of the stats are weighted equally. It is the least useful one since the IV stat value is only added to the base stat, not multiply.
2. You can find CP perfection in app like Poke Genie. This is based on how CP is calculated. Basically CP sees the attack twice as good as both defense and stamina. The problem with this though, is that since defense and stamina both can help Pokemon survive longer, they are not half as important as CP implied. Also, since IV stats are added to each Pokemon and each pokemon has different base stamina and defense, for each mon, those two stats are not equally important for each mon too.
Here i have come up with some examples and those can show how do i choose which one to power up.
Example 1 with Articuno: base stat 192/249/180. Since its attack is lowest, so we should choose 15 attack IV as more important than those two stats. So in this situation, CP perfection should be a great criteria to decide.
Example 2 with Zapdos: base stat: 253/188/180. Since its attack is the highest, adding more Attack stat will have less effect on him than adding other two so both CP perfection and general IV is not a good criteria. Also in this example, Stamina is slightly more important than defense. So you should be more lenient on Attack stat and choose the one with the highest stamina then defense.
Example 3 with Lugia: base stat: 193/323/212. I have two with 15/11/14 and 15/12/13. Since both of them have the same attack stat and its defense is so much higher than stamina, the first is definitely better. And luckily, the first one also has higher CP perfection. So in this example, CP perfection definitely is the criteria to use.
In conclusion: for those mons with lower attack stat, CP perfection is the best criteria to use. As for those mons with other two stats lower, going back to base stat and calculate the percentage increase due to certain stat might be the only way to go. You can still use CP perfection to decide everything, but my caveat is CP formula can be changed in the future and what you have powered up may not be your best choice then.