If there's only one bone to certain vertices, set it at 0.5 (unless you'd like a lower value), but that's the maximum one you should use. When out of the "edit envelope" section, select the box associated with that bone, and move it back and forth. Does the bone move when you move it?
Also, a recommendation when setting the bones, if you move the box out before selecting the vertices, move it by an easy to remember value (20 cm, -15 cm, etc) so that when you finish with it, you just need to type in the opposite of that value to have it back exactly where it was so that you can easily export without the bones being in a different place then the original model. I made that mistake with my first model, and that avoids that issue.