I give delivery drivers 10%.
The take-out place I worked for delivered to a wide area in the mid San Francisco peninsula. Some deliveries might take about eight minutes; some closer to 25 minutes.
Sometimes the orders would be for just a couple of menu items, totaling perhaps $5 or less. A large order might get up to about $25 dollars for dinner for four.
Using your formula, small orders to a distant customer might net about $1 per hour in tips. Large orders to close customers could earn $27.50 per hour in tips.
I would suggest that you look at what percentage of his income your tip makes up. It should probably be between 50 to 100 percent of his wage, taking into accound how much of his time you have used. The actual value of the item delivered is probably not very relevant.