i usually just cheat and cut the back mount a little thinner if I get stuck like that.
I had the same problem the other day..Its like less than half a link too short but if you add 1 pidly link with an extra joiner the thing becomes rediculously too long!!
just cut 5mm off the back mount which hugs the seat post and all is dandy.
Stuff buying a half link if ya dont really need to.
U dont even need to really cut 5mm off. Probly 2mm would do... could even do it with a simple file + a tub of elbow grease.