Another good way of working out your ROD is dividing your GS by 2 and adding a 0 at the end of your answer.
ie, GS of 160/2 = 80 + 0 = 800 feet a min,
GS of 150/2 = 75 + 0 = 750 feet a min,
GS of 140/2 = 70 + 0 = 700 feet a min
GS of 136/2 = 68 + 0 + 680 feet a min and so on.
Seems to work and gives the same answer as the GS x 5 rule. Depends on how good your mental arithmatic is I suppose.