I have forgotten more than I remember about setting those chains.  This I remember, the chains stretch over time, so a set number of links won't do much good if you are setting a used planter. 

As I recall, I made a guess, raised and lowered the markers, and re-set according to how far it was off. 

I remember putting new chains on frequently, seems we always had uses for the used chain on various projects.  Our problem was our tractor hydraulics got faster and quicker than the planter would allow, which was the reason for stretching.  When we used the 56 on M or Super C International, there wasn't a problem.  it was later that the newer tractors really needed a restrictor in the line, or slow the engine while raising.  We did idle down on the ends while turning, which helped.

We did much of the adjusting at the marker, where the slot attached to the marker.  I recall when I did know how many links should fall from the slots, but it has been many planters since I had to know that.