not sure of this but first mob on mon1 is 0 and first mob of mon2 is 10..
if i remember the rule was take the number of the mon file.. (example mon2, take the 2) then subtract 1 (2-1=1) then multiply 10 (1*10=10) then sum the position of the mob in the file subracted by 1 (example third mob in the file -> 3-1 = 2)
imgindex of the 3rd mob in mon2 is ((2-1)*10)+(3-1)=12
the rule should work for all the mon files execpt the dragon.wil and the sabukwalls i guess