If your trying to find out what percent your '09 net sales are over '08 should you subtract '08 from '09 and then divide by '08 sales?

If 08' > 09' sales
'09/'08=# multiplied by 100 = % that '09 sales are of '08 sales
But if you want the growth rate of sales here is the formula to get the exact percentage, positive or negetive.((current month's sales - last month's sales)/last month's sales)x100

Saying percent of sales "over" 08 to me means how much the percent of the increase was, which is the typical way of doing it.

i.e. not this:
"If 08' > 09' sales
'09/'08=# multiplied by 100 = % that '09 sales are of '08 sales"

If you're looking for the percent of the increase or decrease, the simple answer was "yes." If you do it in the order stated, any decreases will automatically work out negative.