I use the caliper technique, measuring inside to inside. I found that the unstretched dimension over 10 links (10? 11?) is 5.2 inch. I found this by measuring the diameter of the rollers new (0.3 inch), and offseting the pitch by the diameter (5.5 inch minus .3 inch = 5.2 inch. Given that, I calculated what 0.75% stretch and 1.0% stretch would amount to, and I made a little table showing % stretch vs. actual measurement. Here it is:
0.50% 5.226
0.55% 5.229
0.60% 5.231
0.65% 5.234
0.70% 5.236
0.75% 5.239
0.80% 5.242
0.85% 5.244
0.90% 5.247
0.95% 5.249
1.00% 5.252
So I measure the chain in 3 or 4 spots, take an average, and look up the % stretch.
I toss the chain if it runs rough no matter what the dimension say.