On Fri, 24 Jan 2014 00:36:42 -0800 (PST), Sonnich Jensen
I was thinking of this. 2" _is_ 5cm, and I was thinking letting the upper and lower 20 cm be free, which gives me 50cm to drill = 11 series of holes (44 holes)
Then again I agree that once in place I will probably never really move the shelves...

For the entertainment centre I built, I drilled holes every inch
within three inches of the top and bottom. My components are varying
heights and I occasionally add, remove and rearrange those components.
It gives the most flexibility. Easier to do it all now instead of
finding out down the road that you need a configuration you haven't
drilled for.