When I am teaching film development, I like to compare time and temp charts to stripes on the road. They are guidelines.
You will NEVER develop 2 sheets or 2 rolls of film identically, The objective is to have an idea of where your errors may be and to have a handle on what you are going to do in development BEFORE any chemistry gets poured. How long is it going to be in each chemical, how long and when you will agitate, how long it takes to pour in and out, where the timer is, where the cap for the developer bottle is...etc, etc. Get your act together before and don't do anything "on the fly"