The gutta percha cone fitting challenge is to have "tug back" of smooth binding at the terminus of the prep and not coronally. A gutta percha gauge or taper plate (Dr. Scott Martin’s Taper Plate in this instance) work great to nudge cone fitting to perfection. This case shows a cleaned and shaped simple lower canine root canal system. The root canal preparation taper was determined to be a .10 almost a .12 and fit a .08 cone (it's a good idea to fit a cone one less than the taper measured with a tapered file).
Using the taper plate, the cut-back .08 cone fit at apical size .37, tactile sensations revealed it was probably binding coronally (ie. feeling a tactile, smooth, broad, "mushy" wedged binding of the cone when not contacting at the terminus). When switching to the narrower cone (.06) and cutting it back to 42 and it fit to length. This is a clear sign that the .08 cone was binding coronally. This was further checked wtih the .06 cone by placing an extremely narrow non-standardized cone in the canal and cutting it back to where it fit to length which was .42 again. The .06 size cone was a perfect size for this case. It filled in the maximum amount of space apically with the cone, yet did not risk slipping through during the downpack.
When fitting cones, do not trust that the apical taper size matches the last taper file check, especially if the apex has been previously manipulated or there might be some periapical inflammatory resorption. Using a very narrow cone and cutting it back to where it binds at length can assure apical tug-back. Simply choose the largest taper size cone that fits to that apical size. In this case it was a .06 (a large supplemental cone can be placed to fill in the excess coronal space to insure smooth multiple waves of compaction during the downpack as described by Schilder and Goodman).