Interesting topic.
These fuel circles seem to do exactly what most of us probably have in the fuel column of our plog. These fuel circles are drawn on the chart(?). My plog for example has a fuel column at the end of each leg with how much I expect to use, and what I expect to have in the tanks. Both written in a box with a horizontal line as opposed to a circle. Now whether these 'estimations' are for the leaned engine or otherwise they work and will do just fine for my flying. Furthermore in the planning phase of the flight 45 mins extra fuel is added plus 10% of that again. So with that in mind I suggest the leaned/full rich figures wouldn't be too dissimilar for the GA hops we see around the UK, and if so the contingency will cover it.
On using the circles method: the red half of the circle seems to be drawn in red to be conspicuous. I would consider drawing the whole circle in black or blue regardless, as red wont be easily read in red light (night flying?). I'll also mention that if flying IFR then fuel circles may not be the easiest way to check fuel calculations. IFR flight = higher workload and tends to be more accurate due to the navigation aids, ATC, etc, and I personally believe the chances of getting lost whilst IFR are close to zero. The latter reason is my preference for not using circles drawn on the chart (as well as preferring as clearer chart as possible).
Just my thoughts
GW