It's not as simple as a side mount drawer where you take the opening measurement and subtract an amount and you have your drawer width.
They have a somewhat simple formula that takes into account the drawer side material thickness to calculate the width of the drawer.
The reason for the formula is Blum is looking for the inside of the drawer dimension, not the OD. I've made up an Excel spread sheet with a formula that calculates the width of a drawer depending on the material thicknesses. I use 15mm Baltic Birch plywood for the most part and it varies in thickness like most plywood does between .580" and .605" so I pop that specific number into my spreadsheet after measuring it with a caliper.
If you are using solid wood it's much simpler to use standard thicknesses and just subtract the required amount according to their chart.
Either make or buy their jig to drill the hole in the rear of the drawer. I've made a router jig to get the notch for the back side. The corners aren't square on the notch, I use a 1/4" flush cut bit and I haven't had any issues. If you don't have a notcher you need to use a tablesaw or a router jig to make the notches. Or just have the back side of the drawer undercut so there is nothing hanging down.
Getting set up is the hard part. After you do that the drawers are much easier to use than a side mounted slide. You are only mounting the slide and then a simple lock under the drawer. No aligning a set of slides to each other.
=C8-(1.656-(C5*2))
Where C8 is the opening width and C5 is the material thickness. 1.656 is 42mm I believe converted to inches which is part of the Blum formula