Well, I’m sure Mlangsdorf used a lot of math when it came to energy and vehicle parts. So, let’s not be quite so dismissive about things not making sense.
However, with movement speed, yes, math and hard numbers are not great. For example, part of your formula involves knowing the distance of any given tile. It’s been said before that tiles are not a set size, but are variable to suit whatever you’re working on. Hence why a squirrel and a human both take up one tile, or how a bike and a small vehicle are about the same length, despite their real-life counterparts being obviously different.
So I can’t speak too much to your exact math because the numbers aren’t precise. I would encourage you to sit in your chair and do nothing for six seconds. It’s longer than you think. I don’t know that I’d cover 27 feet in six seconds at a walk, but if I were bustling (but not running) away from zombies then maybe?