We did these 15 DME arcs all the time arriving from the north at Kingston, Jamaica. It was a lot of fun in the 727 at 250 knotts. I just put my wing on the vor and pretended I was a Ucontrol airplane with a wire streched out from the wing. Staying a few 10ths of a mile from arc was easy at any speed. I agree 2.5 miles at 250 knots is a proper lead time as it worked for me all the time. I just loved the automation in that 727. It was so primitive it made flying fun.
Next I enjoyed the 757/767 era planes and am retired but loved the old stuff.