Hello, Stonks!
Indeed the way you imagined was done a long time ago and we saw that, on average, each +1 on equipment covers for 0,75 damage on total.
There is also one important note that if your total armor is equal an even number, the total value that'll count is the odd number before, so, for example, if your total armor is 29, you actually have 28 armor.
You can see in more details here.