Using whole hours to plot
In fig 8.5 we can see when we plot a 15 minutes, a 30 and 45 minute plot that the water track (heading) has the same bearing and is parallel to the others. This will always work like this as long as the same amount of tidal vector is used. There is no need to complicate things too much and use fractions of an hour when we can keep it simple and use whole hours of a plot.
The golden rule is to have equal sides on the triangular plot. If we estimate it will take one hour to reach our destination, we must use one hour of tidal vector and one hour of boat speed.