Overandgout,
Working out lat & long of one point based on known co-ords of another is very easy as long as your chart has a scale and you have a ruler. Shouldn't take more than a min or two per waypoint.
In any case, it would be good practice to check any waypoints you work out purely using a GPS, on a chart. That way you'll form a mental picture of the route and might also see marked on the chart, some landmarks you can use on the way to make sure you're still on track. Pure reliance on technology has got people into deep trouble in the past.