I'm afraid it's not that simple. As you mention, planes can fly with a fairly low thrust to weight ratio. Obviously gliders can fly with no power at all, so there's no critical lower-limit.
The other issue is that pitch-speed is important - the prop needs to be producing thrust when the plane is moving, not stationary. Measuring static thrust only tells you half the story.
Instead of thrust, it's better to look at power. Assuming that your motor and prop are reasonably efficient (and most are) measure the power going into the motor with a watt meter (or run it for a minute and see how much energy your charger puts back into the battery). Then weigh the complete model, including the battery.
- 50 w/lb is about the minimum for gentle cruising and slow climbs.
- 100 W/lb is 'sporty', say for a warbird. You don't have to worry about conserving energy and can easily climb out of trouble. It might not have unlimited vertical climb though.
- 200 W/lb is unlimited vertical climb, hovering and climbing vertically out of the hover.
For ducted fans, you should double these numbers. i.e. at 100 w/lb you'll need to be careful to conserve energy.
However foam board planes aren't super-efficient or super-fast, so you can usually do a simple thrust test to check it'll be OK. Dangle it by a wing tip and gradually apply power.
- If the model has enough power to support its own weight, it's probably around 100 w/lb
- If it can only reach about 45 degrees, its about 50 w/lb and will fly if you're careful.
About pitch speed
If a prop has 6" of pitch and is turning at 1000rpm, under no load it would advance 6000 inches per minute. Or to put it another way, if the model reaches that speed, it won't be producing any power!
- 'Slow' props have a pitch of about half their diameter (e.g. 8x4")
- 'Sport' props are 2/3 to 3/4 of the diameter (e.g. 8x6")
- 'Speed' props are 'square' with the pitch equal to the diameter (8x8)
If you find your plane accelerates well from the launch but has a low top speed, you might need a higher pitch prop.
If it doesn't have enough thrust to take off, go to a larger diameter.
If you change the prop, you should measure the current drawn to check you're not exceeding the limit of the ESC. Also check that the motor and battery don't overheat.