There is no definite answer. If I see a flashing bar on my gauge, I worry if I am not close to home port.
Remembering that the tanks are not symmetrical a change of one bar does not relate to an absolute quantity of gas. In addition, there may be gas in the tank that sits lower than the lowest level of the sender. Whether or not you can use that gas depends on the height and position of the pickup and the angle of the tank.
The really tricky part is that unless you know when the number of bars changed, you don't know where in the range of a bar the level is.
For example, on my 208 the bars will change from two bars to one solid bar when there is about 20 gallons in the tank. When the single bar solid bar changes to blinking, there is about 10 gallons in the tank.
In this example, note that even though there are 8 possible bars on the level display, a single non blinking bar means I have somewhere between 10 and 20 gallons ( approximately).
The display will also read higher than the real level when on plane and cruising since the hull is angled bow up a bit.
When the boat comes off plane and settles down, my level can change by 2 or more bars.
The moral is: You can't get an accurate display of tank level due to the physics of the system I always add at a minimum the amount of gas I expect to burn that trip.
Fuel management systems are more accurate as long as you enter the correct data.
