you need to reach an average of 2.75.....this is done by dividing the front by the rear.
so working it backwards you need to go 17 x 2.75 = 46.75.
so you need a front sprocket as near to 46.75 as you can get....but gregg.....do yourself a favour....by a 16t freewheel and get a 44 front, its much easier.
lol
Dave