To have to reset the anchor watch at every wind or tide shift is what
I
want to avoid. I want the anchor watch to call me only if the anchor
drags.
If I can position the center of the circle to were to the anchor is,
the
anchor watch will call me only when the anchor actually drags.

**********

Why get so complex?

A properly set anchor will typically hold against minor shifts in the
wind. If current swings you around 180 degrees, the anchor may pull
free for a second before resetting but in most situations should not
drag.

Consider setting a depth alarm on your sounder. Pick numbers a couple
of feet less than you will have at the lowest point in your intended
anchorage cirlce at the lowest tide, and a few feet deeper than you
will experience at the predicted high. Assuming you are going to anchor
in the shelter of an island or a bay, (rather than in the middle of an
undersea plateau), depths should vary quickly enough outside the
intended circle to alert you that you are moving.

Another low tech solution is to drink lots of liquid before retiring.
If you don't post an anchor watch, (and who really does 99% of the
time?), the rule is that anybody getting up during the night to visit
the head is also required to verify the the anchor continues to hold
and the boat is not drifting. :-).