Basic questions about valve shims

[Crimson]

Apologies if this is a stupid question. Working on this bike is my first experience of a shim-over-bucket valve adjustment system.

Haynes has a list of valve shim sizes corresponding to valve shim measurements. This means, you can measure how much clearance you have, and if it's out of the allowed bounds, the chart will give you a three-digit number npq for a valve shim of size n.pq mm to fix the problem. But why should you need this chart at all? For example, I measured one exhaust valve clearance as 0.15mm. So, if I want to change that to 0.25mm, all I ought to need to do is the following:

1) Pop out the valve shim that's already under the cam lobe.
2) Read the number (npq) under the shim.
3) Buy a new valve shim with the number (npq-10) written on it.
4) Put the new valve shim in where the old one was.

This should result in the new clearance being 0.25mm? Have I understood it correctly? Thanks