Hello, you say that you got this problem after you topped off the coolant?
My gauge(85 westy GL) read low by more than a needle width for a year.  One
day the coolant level sensor blew out of the expansion tank(I have heard of
this happening before( one more way that you can loose your coolant before
its time!)). I replaced this sensor and low and behold, as a side effect of
this repair, the gauge needle was now back to a normal position. I have to
tell you that I knew that would happen(this is a lie).However, upon closer
inspection of the level sensor, I saw cracked plastic in one of the probes.
Perhaps this sensor is causing your problem. I say this because you added
coolant  and this problem began. Maybe the electrical signal was  somehow
altered with the additional coolant.