you are comparing 2 things. if you are trying to determin how fast a specific person can get from one place to another using the same technique, then that will always be different according to how fast a persons reaction time is. what we are determining however is the speed of the technique used without any involvement of the user except for initiating the techniquePretty much. to the victim/opponent during the entire process nothing would change everything will seem 'frozen'. Imagine someone shining a flashlight in your face. For those photons to reach your eyes it takes time but to you eat seems like its just there. Now replace these photons with a body; as weird as that sounds. Say the light turns on as soon as the ninja starts moving. You would be dead before you realize that the light was turned on since he would be moving 'faster than light'. The ninja would reach you before the photons of the flashlight did.
However FTG is instant like Zaru said so the terms speed and velocity do not apply. The only thing that applies is the time it takes for the users brain to process the action of casting FTG.
in the way your saying it, minato will be able to go anywhere in the universe with a mark at a speed of probably 0.25 seconds.
thats like saying if 2x = y with 2 being the speed of ftg and x being the user and y being the speed travel altogether, that x will always = y since the travel time altogether is based on how fast the users input is reguardles of the constant.