Joined: Jan 2008
Posts: 23,643
Likes: 1,107
From: Mans of NC & SW UT Desert
Bikes: 86 Katakura Silk, 87 Prologue X2, 88 Cimarron LE, 1975 Sekai 4000 Professional, 73 Paramount, plus more
+1 Your chain is very likely too short, unless it was too long when you started. So it has to go. A worn out derailleur should be dealt with as well. Easiest to do both at once.
Using old chain as a gauge? Well, if it is too short, that really won't work.
I buy 5/6 speed chain for under $5.