If your going to divert form one place to another, then (in real life) theres probably a reason.
Lets say you want to go from liverpool to birmingham. If you file a flight plan, you will include a diversion, so in the pre flight planning you will already have an idea of your timings and headings to fly.
But on the test you'll probably divert from a turning point, to a church on the side of a hill thats covered by trees and is only visible from the opposite direction(i dont think my examiner liked me, or he was taking the piss, i was fairly sure of the surrounding terrain, but couldnt see the church, a quick 180 turn and i found it).
anyway to cut a small story short.......fly the plane, hell probably tell you the diversion a few mins before. so get the map out, draw a line, get the track and distance. (one of those rulers with the compass rose on is great). look at the wind arrow on your map, and if it was me, take an educated guess as to how the wind will affect you. (1 in 60, works a treat), if you know your tas, and from the h/twind your gs, you can your out your time.(miles per minute, 120kts = 2 miles per min and so on.
if its a long diversion, then after the first check point, youll know if you were right or not.
practice on the ground, after a while, you can draw a line on a map and guess the heading and distance within a maybe 5 degrees and 2 or 3 miles.......
or somthing like that.........