Banks lend money most eagerly to people who do not need it. And having just one good rule ( follow your good judgment) is great, so long as you have informed good judgment. I believe the best rules are made for people who do not necessarily have an innate sense of how to do it. Once the rules have been internalized, it is much easier to go by the seat of one's kilt. Some will suggest that these rules are outmoded, but that is the nature of tradition- it follows the past.

Without further ado, here are a few good rules:
1) Do not be overdressed or underdressed.
2) Be completely dressed
3) Be consistently dressed
4) Be comfortably dressed

Again, these rules are probably best understood by those who have followed them so long that they don't need them, but I will go a little further.

Rule 1 means to wear a tie most of the time, but to match your formality to the occasion. Within reason, it means to dress like everybody else.

Rule 2 means to wear socks and shoes and a belt or waistcoat and a sporran - and a tie if most people there will be wearing one.

Rule 3 means to wear formal clothes throughout or casual clothes throughout and don't wear a casual garment with formal ones or vice versa.

Rule 4 means to avoid being self conscious, but also to wear clothes that fit your body. And remember who is in charge ( you, not your clothing). And wear it like it is clothing, not a costume...

You will notice I didn't say anything about colors or collars or matching or ruche ties. Oops, until now.

Sure, just follow your heart, but make sure your heart knows where it is going. As irritating or frustrating as it may be, the source of these rules is simple- it is how it was done before you noticed. Nobody is going to MAKE you follow the rules and if you dislike them, you are welcome to ignore them, but the rules (aka tradition) are the rules and you can't play the game without them. If you try, you are playing another game. And of course, that is your business.