Ferg has drawn a graph of tank capacity against fuel depth, which appears below.
This was done for a tank dated 13Jan97. Some earlier tanks were larger than this. It is not known if current tank deliveries are the same size as this.
To quote Ferg :-
Somewhat like Tony K's method, I took the fuel tank to level area in the sun (cruise attitude) and glued a metric measuring tape to the back. I then added water, one litre at a time, and each instance recorded the depth in mm. This went on until I had reached the bottom of the vent neck. I figured everything above that was gravy. I then drew a graph of mm [vertical] vs. litres across the bottom, and joined the dots.
Herewith is my graph - you will see that it corroborates the 'saddle' contents as 11 litres per side, and full tank [bottom of vent neck - 356mm] as 58 litres. The graph shows two figures at each measured point total and main only - differing by 11 litres in starboard saddle.
Ferg is at VE3LVO@rac.ca