Examples of B/C/D calculation:
I’m going to use the current 1.9.x max titan HP value for a 10* which is 3234000 HP. This math holds for all titan levels, just use the HP of the titan which you are fighting as the value for T-HP (Titan HP).
B: T-HP * 0.033 // 3234000 * 0.033 = 106772; higher than this and in ranks 6 or lower, will result in a B grade. Incidentally 3.3% is roughly equivalent to 1/30 which would be the average damage needed per member to kill a titan assuming a full alliance of 30 members.
C: T-HP * 0.01 // 3234000 * 0.01 = 32340; higher than this but below the B line (106772 in our example) will result in a C grade.
D: Less than the C value, 32340 in our example, will result in a D.