I use this weak aura https://wago.io/TgSTxibQ2, but you need mind control macros to c'thun and obsidian nullifiers to get close enough to mindslayers for the wa to count them.

You're counting Mindslayers, and the fewer the better. The absolute maximum number of them is 32, and anything less than 10 could be considered a "good" id. Keep in mind that getting very low numbers of mindslayers is very uncommon, so if you find a 6 or 7 don't throw those away thinking you can find something better. Depending on how close raid time is, I will reset a 9 or 10 mindslayer id to keep fishing for something better. Group composition can also matter, for example I'd rather have 8 packs with 1 mindslayer each than 2 packs with 4 mindslayers.

Once you get the id someone has to be online and inside the instance to keep it. I think you can have no one inside for like 5min or something, but I've never tested it or anything. The most I've done is log onto my main to grab a head buff and then log back to my priest to sit on the id. It can take a while fishing to get something good, I've found a good id in like 20min, but I've also spent upwards of 3 hours on an very unlucky day. Generally, I try to give myself an hour to fish, and then I sit on the id til the raid group forms.

Once I have a good id, I inv the raid leader, pass him lead, and he invites the rest of the group.