When coming onto a thread asking for help, it should be expected that you will be scrutinized to the utmost degree. Some folks will appeal to their personal epeen and emit an internet tough guy approach, others will look at the facts like your logs and be straight up. Preferably the latter, even if it is harsh criticism. Asking for help in a forum, one should have the tough skin to sift through it without reproach. With your history in the forums, it should be easier to recognize those that will give facts and assistance, despite how negative that can be at times. I applaud you for trying to do better, so don't let one declined app sour your juice box.
To answer your OP question, it's one giant math equation. Basic math for the most part. Constants to consider are fight duration, GCD's avail in that time, CD durations, natural focus regen, CS focus regen, etc.
Consider a 5 min fight, which is 300s. That would equate to 300 GCD's. With latency and personal ability to push buttons really fast, that's not really possible. But just follow me for example purposes. Of those 300 GCD's, as BM, 50 should be KC if used immediately when off GCD(300/6s). 50 KC at 40 focus per is 2000 focus needed for the fight duration. That is just one of the GCD's used out of the priority list used during a fight, but as your number 1 priority shot, it's where you can start the equation of what % of a GCD you should be hitting for a fight. Rounded, that is 16.6% out of 100% of the fight should be just KC. Keep in mind this is not to be confused with 16.6% of your damage. Read it as uptime of the "spell" on the target.
So with 250 remaining seconds, you have other GCD's to use and CS to regen focus needed outside of natural focus regen. The equation requires you use all of the data in the logs regarding your CD's, focus regen, BW, etc as your variables. Like I said, it's a giant math problem, and one to use after a fight when looking at your log. That way you can see if you did too much, or too little of an ability during a fight. It's not for everybody, but it can provide the answer to your OP.