The RAW is:
Sanity Score, Edge, and Thresholds
Each creature has a sanity score, along with a sanity edge and a sanity threshold. These values depend on the creature’s current ability scores and ability damage. Increases and penalties to ability scores (even temporary increases and penalties) adjust these numbers. Each discrete instance in which a creature takes 1 or more points of sanity damage is called a sanity attack, regardless of what caused the sanity damage.
Since effects that deal sanity damage are always mind-affecting effects, mindless creatures are immune, and do not have a sanity score, sanity edge, or sanity threshold.
Sanity Score: Your sanity score is equal to the sum of your mental ability scores (Charisma, Intelligence, and Wisdom) minus any ability damage taken to those ability scores.
Sanity Threshold: Your sanity threshold is equal to the bonus of your highest mental ability score minus any ability damage to that score (minimum 0). When you experience a sanity attack, if the sanity damage from that attack equals or exceeds your sanity threshold, you gain a madness, either lesser or greater depending on the relation of your current sanity damage and your sanity edge (see below).
If your sanity threshold is 0, you always suffer a madness upon taking 1 or more points of sanity damage.
Sanity Edge: Your sanity edge is equal to 1/2 your sanity score. When you experience a sanity attack that causes you to gain a madness (see Sanity Threshold), compare your total amount of sanity damage to your edge to determine the potency of the madness. If your current sanity damage is less than your sanity edge, then you manifest a lesser madness.
If your current sanity damage is equal to or greater than your sanity edge, you manifest a greater madness instead.
Furthermore, when you accrue total sanity damage equal to or greater than your edge, any dormant lesser madnesses you have manifest again.
So if I’m reading this correctly the score is the combination of the 3 mental scores (not modifiers), the threshold is the the highest modifier of the three and edge is half of score.
So if I have a level 15 character IQ: 23 WIS: 8 CHA: 14 the stats would be:
Score: 45
Threshold: 6
Edge: 22 (rounded down from 22.5)
If said character took insanity damage of 6 or greater it would receive a madness, lesser it under 22; greater if over. Is this correct? i think the edge is throwing me off
That is correct. Also remember that Sanity Threshold works kinda wonky with subtracting Ability Damage and whether that applies a madness or not. For example you have 23 Int, so your Threshhold is 6, but let's say you've also received 6 ability damage to your Intelligence (so your Int score is temporarily reduced to 17), and now your Threshold is 3 (current Int Bonus) minus 6 (ability damage to Int) = -3 temporary threshold (so it's minimum 0). So any sanity attack would cause a lesser madness while having 0 threshold. And once your Sanity damage reaches 22 or higher, you would always gain a greater madness.
So due to the highly oppressive nature of Sanity rules and the significant difficulty with removing the effects of Sanity loss, I do not recommend playing with Sanity rules for parties below level 12, with characters that have their highest Mind Ability Score Modifier as lower than 8, and/nor with a party that has no access to restoration, greater restoration, and/or wish/miracle spells.
It's simply not fun when the party has no reasonable counters to this other than "sleeping it off" with counseling sessions, and when the party doesn't have the levels/gear to have high modifiers for their mental ability scores to mount an adequate defense against sanity attacks.