It is often helpful to hear something explained more than one way.
Let’s say you measure a particular exhaust valve lash clearance to be .12 mm. You decide that you want it to be .20 mm. Therefore it is off by .08 mm (20-12).
You remove the existing shim and see that it has 273 printed on the underside. 273 means that it is 2.73 mm thick. You need a shim that is .08 mm thinner.
2.73 mm – .08 mm = 2.65 mm. You need to replace the old 273 shim with a 265 shim.
Remember, when you install the new shim to put the “265” on the bottom so that the cam does not wear off the “265”. If you forget, you will be mad at yourself next year when you can’t read the number!