The chart was specifically designed to minimise the errors associated with plotting radio bearings. Scale Deviation has been designed not to exceed 1% and is achieved by two factors:
The ratio of the spacing of the SPs to the Latitude range of the chart does not exceed 1:6
The maximum spacing of the SPs does not exceed 14 degrees
If these parameters are applied, the SD will not exceed 1% and any variation between a straight line on the chart and a great circle will not exceed 0.75 degrees and can therefore be considered constant. Not exactly O level maths!
(Ref Maps and Charts - RAF College of Air Warfare) probably the same source as the Bristol notes.