From Worms Knowledge Base
There really is no maximum fall damage — there's a maximum vertical speed, and it's normally at this impact speed that the highest fall damage is reached. With TestStuff5, there's no maximum speed. The maximum fall damage with TestStuff5 should be 32767/18 = 1820 (at extremely high speeds the damage will be pseudo-randomized due to integer truncation, but it'll always be <= 1820). 
Fall Damage Formulas
Here is the exact formula, in BASIC parlance:
FDCOEF = (FDBYTE * 50) mod 256
DAMAGE = INT(((VSPEED - 8 + 1/65536) * FDCOEF + 18) / 18)
VSPEED is the vertical speed at impact in pixels per frame. The normal maximum for this 32. So using the standard values for everything: DAMAGE = INT(((32 - 8 + 1/65536) * 50 + 18) / 18) = 67
Note: With a value of FDCOEF=0, this formula gives a DAMAGE=1. However, W:A treats this as an exception — FDCOEF=0 disables fall damage completely.
Here is the reverse formula, covering values of FDPERCENT ranging from 0 to 508 in increments of 4: FDBYTE = ((FDPERCENT / 4) * 41) mod 128
Before Deadcode's fixes, W:A used to use this formula instead: DAMAGE = INT((VSPEED - 7) * FDCOEF) / 18 This yields a standard max fall damage of 69 instead of 67.