Okay now. Where are we going with this?
Several problems have been solved when too much light was present.
Now possibly not enough?
So I have to ask, are shop fluorescent lights okay?
How about an overhead LED bank? (which actually flicker even though you cannot see it).
TL DR: What is a recommended room light source and how strong?
I had always assumed I could solve the ambient light problem by just covering the top of the Forge during startup. Apparently not. Maybe if a white mat board were used, it would reflect enough light back in?
I’ve got overhead fluorescents in the office, bright ones, but the machine is offset so that it is not directly underneath them - I’ve never had a problem. 
The one time I had calibration trouble with sunlight coming in and hitting the top of my Glowforge I stuck a piece of Proofgrade acrylic on top and it worked fine.
I have 8’ fluorescent fixtures, one just offset from directly overhead. No problems.
The single 60watt incandescent bulb in the ceiling fan of the room was enough to trip up the calibration of my machine. When I turned off the light (or placed a piece of cardboard over the GF thus making it completely dark in the unit) it calibrated fine.
However as of a week or two ago I am not experiencing any calibration issues at all regardless of light levels.
But no mention of an update that might improve calibration here: https://glowforge.com/latest-improvements
It is a crazy situation people wondering around in the dark trying to work out if they have fixed a problem themselves or GF have changed something. And no indication from support what bugs there are in the calibration routine. All they say is put the head under the camera. Some people say that always works, some people say it always fails and you need to put it in the park position.
Some times it is too much light, some times too little light. Sometimes a square cutout on the bed throws it off, or does it? Nobody knows a definitive way to make it home reliably. Some people never have a problem.
Now if this was a proper software project there would be a list of bugs and proper release notes and all this uncertainty and confusion would go away. I am sure GF staff know exactly why the calibration fails but they never let on.