Rare Candies and You?

Hey guys so I have a bunch of rare candies and plan on using them on the following:

A. Treecko: I recently got a lucky 14/15/15 and have yet to register a mega Sceptile. It’s a good mega counter option currently for Mega Gyarados. Sceptile is also one of my top 3 favorite grass starters.

B. Torchic: I have a lucky I recently acquired 14/14/15. I have yet to register a mega Blaziken into my Pokédex. I know Mega Blaziken will be a top counter for Mega Abomasnow and Mega Glalie. Plus Mega Blaziken is a top fire and fighting pokemon.

C. Lucario. I know it won’t be appearing on Pokemon go anytime soon. I have a 15/13/15 and plan on giving it a second moveset to use as a top steel and fighting type mega/Pokemon overall.

I do have 2 questions for yall:

1. How do you guys decide to use your rare candies? I do have rare legendary and mythical pokemon tagged for both rare candy and rare xl candy.

2. At what point in saving candies do you choose to use them? Like at what point do you go like “Welp I’ve saved enough rare candies now. Time to use them.”