I think whenever something is somewhere marked as finished (that is moved to "Completed Items"), "progress" should be set to "100%".
It looks strange now, have a look:
When you move it back to "To-Do Items" (mark as not finished) it should ideally re-load the previous (pre-finished-checked) value. If this is complicated to implement, it could be set to zero, or offer an dialogue to set a new value.
What do you think?