If it works for what purpose I intend it to work, then I feel that's a success.
Doesn't always have to be for group play either.
If I'm looking for a speed build to farm mats with, and it works the way I intended it to work, that's good theory crafting.
A good start is anything that doesn't revolve around "if we don't use [insert generic FOTM build here] then we're not viable because [insert YouTuber] says we will do 4k less on our 80k+ parses, fix it NOW Zos!".
The flip side is that bad theorycrafting is easy to point out: "I want to wear heavy armor and dps with a Resto staff, why do I keep getting kicked from groups?".
I don't give a rat's bottom about someone else wordsmithing a long elaborate talk about whatever they think better and some associated number crunching. I test stuff, fail, win, adjust, rince and repeat, and in the end I get a working build.
Never understood people who watch videos about how to play a game when they can play the game.
So, "good theorycrafting" is "none at all, thank you very much, I'll experiment".
Make the gear, up to at least blue quality, test it doing what I normally do. If I feel there is an improvement or it works, then I upgrade it to purple, give it decent enchantments and continue testing.
Only if it "seems" to perform better than what I was using before do i bother upgrading to gold.
If its dungeon gear, farm the pieces I want, run through the same tests, if it meets my standards then I re-trait any odd traited pieces and upgrade.
If the gear turns out to be crap it gets deconstructed, if its just adequate it gets banked for an alt. But for the rare gear that's good it gets used.
Then again got plenty of gold to actually make and test stuff out, why bother just looking at set bonuses and numbers, when you can test out the real thing.
Doesn't cost a fortune to farm the mats and ask a crafter to make something for you, most will make something for you if you know what you want and have all the mats ready to go for a small tip.
If its dungeon gear, queue with a tank or healer in group when farming a specific dungeon. then just upgrade the pieces, if your at the point where your experimenting with gear, you should have the equipment crafts to 50 not expecting people to have researched all the traits.
I pay quite a bit of attention to theory crafters - some set ups are objectively better than others imo and, personally, I wouldn’t find it fun to find out which is which.
Their builds and CP set ups have been tested AFAIK and my experience is that when they say a set up will produce legit end game results, that’s what happens.
I’m sure there are alternatives and tweaking would help but, yeah, I do keep tabs on their thinking.
Andymac - Magicka DK - EP Grand Overlord - Flawless Conqueror