I've always used the 1 in 60 rule.
For instance the arc to ILS at Inverness rwy 05 is a 13 mile arc and the lead radial is 10 degrees prior to the inbound.
On a 12 miles arc each degree is 12/60 miles = 1/5 th of a mile. Thats near enough 13 miles for me to make the sums easy.
10 degrees would equate to 10 x 1/5, approx 2 miles before the radial required.
2 miles before the radial seems a good cut, if you left it any later you could risk going through radial