One thing I like about the Thimblelady cone thimbles is that exact fit is not crucial. The thimble has an open top, so length of fingernails doesn't matter. The thimble is meant to cover the tip of the finger, including the end joint.
During the day, as my hands swell or shrink, the thimble just sits a little higher or a little lower on my finger. There's always at least four dimples that are in the right place for me to use comfortably.