There's only 1 way to find out how far the reserve will get you.... ride the bike till it stops, see how far you got on the reserve trip.
Of course you'll need some petrol now, either take some with you or have a friend/mate/significant other bring you some.
When I did this I got over 30 miles on the reserve trip, and I rode it hard till it run out, so I know if I get caught short, if I ride it a bit smoother, I can get more miles than that!
Lee