Well, any DME arc that I have flown has had a lead in radial. But if I ran across one, I would probably give a 45 degree intercept. For example---flying 270 on the arc, inbound course 360. Turn right to 315 to intercept. Seems like atc likes the 30-45 degree intercept range. Just trying to mock what they do.
I think what you are actually asking is at what point should you leave the arc to turn inbound for the final approach course.
I find when teaching students that if you lead it in by 5 degrees off of the vor that that should give you enough time to intercept the localizer ie. 330 degree final approach course, when the vor course deviation indicator centers up with a to indication on the 325 radial start your turn to a heading of 330. Does that clear things up?
There are actually quite a few without lead-in radials.... Humboldt, Tennessee (M53) doesn't have one, nor does the VOR rwy 23L at Knoxville/McGHee-Tyson (KTYS). I've seen alot more, but those are the only ones I can remember off the top of my head.
Speaking of crazy arcs....anybody here ever flown into the Martin State Airport in Baltimore (KMTN)?? It's kinda wild....there's a VOR approach that has the final approach course along a DME arc....FUNKY!!!!
I am usually in a Seminole when IFR flying at 110-100 KIAS. I usually lead the turn by 10 degrees. When the CDI centers up 10 degrees prior to the final approach course, I set the CDI to the inbound course and turn to intercept it at a 30 degree angle.
Since your distance from the vor varies on different approaches, the lead radial will vary also. You also have to take into account your speed. A good question is who is the Lead Radial calculated for(regarding speeds)? A 737 or a PA28?