mattyj and vne,
Its not 10% nor 1%, however the example that you give using 140kts is correct, that makes it 0.5%.
Ie, With 140kt groundspeed 0.5% would be 0.7 DME, hence your lead distance for the arc. This works regardless of the angle of intercept. If your approaching the arc at an oblique angle your groundspeed will be less hence the lead in distance is less. Becareful the groundspeed has to be read off the DME not a GPS.
Hope this helps.