OOOoooo - this is such a great question! One of my favorite things to do is play with creating variations. I am hard at work on a book about flourishing and a large section will be variations of both capitals and lowercase for each letter.
Sadly, I did not develop a system to track all the variations I have seen over the years. They are scattered about in my studio on various scraps and pieces of paper! But I am working on something to remedy this that I will share at a later date.
In terms of knowing when it is appropriate to use a variation - let your piece be your guide. If you are doing something that you want to be strictly a historic Engrosser’s Script, then I would not use variations. But there are no calligraphy police and honestly, you can use a variation whenever you want. I would just advise not to get carried away in terms of flourishing as you want it to add decorative embellishments but not distract from the overall piece.
I was just playing on ProCreate so did some variations of F for you. Keep in mind, these are digital calligraphy so may not translate well to pointed pen. But they should give you some ideas in terms of how to vary your letter forms. PLAY! See what works and what doesn’t.
