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.