Rule of thumb. Take your cruising altitude and multiply that by 3. That will give you your distance in nautical miles for your TOD. Calculate the distance not from airport but your GPS waypoint that vectors you onto the localizer and you should be all set. I use this all the time and it works like a charm.
FL340 x 3 = 102 NM from your waypoint for the localizer.
Also, welcome to IF! I flew Allegiant out of KLEX 3 flights today to Memphis, Punta Gorda, and Las Vegas.
I lived in Lawrenceburg for a few years before moving back home to Fort Myers in Florida. Very familiar with the area. Cheers!