I've always used a lead point of 1% Groundspeed, for 90 degree turn...200 GS = 2 miles, 300 = 3 miles. On 15 DME arc (4 degrees per mile), at 200 GS use 8 degrees lead, at 300 GS use 12 degrees. On 12 DME arc, same formula yields 10 degrees for 200 GS, 15 degrees for 300 GS, interpolate for different speeds and arc DMEs.