Calculating arrows for British keeps

Keep, Yeomen, Arrow slits, Fletching, Bodkin, Bracer, Chemistry (so expensive!)

8+2+3+1+1+1+1= 17

a)Elite Longbow = 7+4= 11

b)Arbalest = 6+4= 10

Is it as simple as 1+(XD÷17) rounded down? Where X is #of units. D is their damage.

a) 1+(5*11÷17) = 1+(55÷17) = 1+(3.2) = 4

b) 1+(5*10÷17) = 1+(50÷17) = 1+(2.94) = 3

Is the value rounded down or to the nearest whole number? Because it's such a shame if it's rounded down for the Arbalest. Would actually be the same damage output.