Wolf, i'm not sure if this will apply to your 300 , but i recently put white face gauges on my 190 baby.
Basically after getting the cluster out , and removing the instruments, use a fork ( preferably a plastic one so you'll not scratch ) and easily pry it off. It will take a bit to do it , don't use too much force , take your time and eventually will pop out.
Now when it comes to placing it back , put the needle on top of the axe and gently tap it ( preferably with the handle of a plastic screwdriver).
When it comes to calibration, well i can post for the 190 , but i'm not sure it will help you.
In any case ( this applies to the 190 ) once you have the intruments out, will notice that as you move the needle it has a "min and max" position ( you can't pass those without getting resistance ). Make sure the "max" position will be next to 3 ( again this is for the 190, don't know if the 300 D has the same layout). Tap the needle in that position the let it free. As you will play with the needle will see that it will go to 3 ( as max position) and when you let go to the needle will go down to 0
I hope it helps.