For leading a wingtip (90 deg.) turn, two rules of thumb are popular.

The abovementioned 1 percent of KTAS expressed in naut. miles.

Your naut. miles per minute less one. I.e. 180 KTAS is 3 miles per min less 1 equals 2 miles lead. At 240 KTAS, 4 miles per min less 1 equals 3 miles lead.

The first is a civilian standard. THe second prevails in the USAF so I hear. About the same really. Nothing more complicated will survive the 5 years interval between your instrument check and the first DME arc you have to do in the real world.

Some of these formulas are a bit complicated for inflight computing.

I use a few rules myself bit I like to K.I.S.S (Keep It Simple Stupid/Silly)

Arc lead distance = 1% ground speed 120kts = 1.2Nm (remember all turns during I.F should rate 1 turns (3*/sec)).

Turn 90* and if the airfield has a NDB co-located and you are equiped with a ADF then maintain the ADF 90* at all times.

If not ADF equipped, maintain as close to Zero groundspeed as possible on DME readout. If groundspeed increase's then you are moving in/out of the arc. Check the distance readout and adjust to suit.

All Arc's should have a "Lead Radial". I set the limiting radials at the bottom of the No.2 VOR and after crossing each published radial I set the next one. When crossing the Lead Radial set the inbound at the TOP (No.1 VOR should already be set up with inbound during/after the turn-on to arc)
of the No.2 and when 5* (1/2) deflection is acheieved start Rate 1 turn onto inbound.

K.I.S.S K.I.S.S

CB

Some of these formulas are a bit complicated for inflight computing.

I use a few rules myself bit I like to K.I.S.S (Keep It Simple Stupid/Silly)

Arc lead distance = 1% ground speed 120kts = 1.2Nm (remember all turns during I.F should rate 1 turns (3*/sec)).

Turn 90* and if the airfield has a NDB co-located and you are equiped with a ADF then maintain the ADF 90* at all times.

If not ADF equipped, maintain as close to Zero groundspeed as possible on DME readout. If groundspeed increase's then you are moving in/out of the arc. Check the distance readout and adjust to suit.

All Arc's should have a "Lead Radial". I set the limiting radials at the bottom of the No.2 VOR and after crossing each published radial I set the next one. When crossing the Lead Radial set the inbound at the TOP (No.1 VOR should already be set up with inbound during/after the turn-on to arc)
of the No.2 and when 5* (1/2) deflection is acheieved start Rate 1 turn onto inbound.

K.I.S.S K.I.S.S

CB