The following measurements are suitable for the drum fuel tank fitted to Freeman 22's and 23's, and others, with a nominal capacity of 15 gallons. It actually converts to a little over 68 litres.
I do not claim that it is accurate. Merely that it will give a good idea of the number of litres in the tank.
Depth
Litres mm Increment
68 380 26
66 354 16
64 338 13
62 325 12
60 313 11
58 302 11
56 291 10
54 281 10
52 271 9
50 262 10
48 252 9
46 243 9
44 234 9
42 225 9
40 216 9
38 207 8
36 199 9
34 190 9
32 181 9
30 172 9
28 163 8
26 155 9
24 146 9
22 137 10
20 127 9
18 118 10
16 108 9
14 99 11
12 88 10
10 78 11
8 67 12
6 55 14
4 41 16
2 25 25
0 0