First, wrist to floor does nothing but tell you what the wrist to floor measurement is. In other words, it has no bearing on how long a club should be for someone. I don’t know why some companies and fitting methods continue to use it. A well trained club fitter should always determine the length for a player based on the players stature, and what length puts them in the proper position at address and what length enables them to hit the ball the most solid for their ability. You are correct that, generally speaking, the length fit for a person with longer arms might be shorter playing lengths and visa versa for the person with shorter arms. However, it is not a given. You must evaluate the length that allows the player to hit the ball consistently. Also, and I always advise this to our students in our fitting schools, you must “look” at the club / player relationship at address and evaluate it the proportions of the club length to their stature look right. Too short a club for a tall player may make them bend over too much. Too long a length for a short player can be difficult for the player to control and/or hit solidly. Using impact labels, which is one of the best fitting tools a fitter or individual can use to help determine the optimum length for a player, is something we do on every fitting we do.
I can’t really advise o what length is best for you without actually fitting you. If you have played before and have a set or club that is a comfortable length for you, start there. If you want to try to go 1/2″ longer because you are moving from a steel to a graphite, that is OK, but I prefer to fit the length to the player, no matter the material the shaft is made of. A very general guess is that at 6 foot with an arm length that is not unusually short or long and that has a good traditional set up position, the standard length 7 iron would be a place to start. If I was fitting you I would start their and after hitting shots with the standard length, I would hand you 1/2 longer, 1/2 shorter and evaluate if there was improvement at the different lengths, both in performance and feel. That is the best way to dial in the proper length.