While you are at it, make sure you have at least four 5.1 mm valve pads. You will want to do at least one side at a time, and the 5.1 mm pads are useful to determine the pads you will buy to get the pre-load correct.
Here is a good post on the subject:
http://www.peachparts.com/shopforum/showthread.php?threadid=53926&highlight=hlc
Here is a quote about my technique:
"The guage gives an indication of the preload, but that measurement is relative to the thickness of the hockey puck. So if a correction is needed, the thickness of the hockey puck will need to be determined so that the appropriate thickness for correction can be obtained.
Hockey pucks (actually called valve pads or thrust washers) come in seven thicknesses in .35 mm increments with 5.1 mm as the mid point.
Example. The guage shows an additional pre-load of ~.4 mm is needed. Remove the puck and "mike" it. It is a 4.75 size. Obtain a 5.1 puck to correct.
Be aware that the pucks can be pitted badly where the rockers contact them and take this into account when determining the correct replacement thickness.
Also, once a HLC has been unloaded by removing the rocker, it can take some time to return. Wait at least 10 minutes to re-measure. Once I made the corrections on mine, I let it sit overnight and then remeasured and found one lobe where a thinner puck was more appropriate.
If you find you need to go below 4.4 or beyond 5.8 mm to correct, you should probably check that valve and HLC out closely. Herr Fuchs told me that he has only used one 6.15 puck ever.
Cheers."