Here's what I do:
1) Length of each horn from the base (hard to do while horns are still attached)
2) Divide the length of the longest horn by 4 - this is where your four mass measurements come from. For example - your dad's head has 15.75" length, your mass measurements will come every 3.9" up from the base plus the base mass.
3) Length of prong - the length of prong is measured from a point on the back of the horns where a straight edge (ruler) touches both horns. Around from the back of the horn to where the prong starts to branch off of the horn - then follow the upper edge of the prong to the end.
4) Add everything to get your gross score.
5) To get your net score, add all of the differences from one horn to the other on each measurement (length on left vs. length on right, base mass, mass at each circumference, length of prong) all differences need to be totaled. Subtract the total differences from the gross. That's the net - some say net's are for fish!
Depending on the mass measurements, I would say they would score in the low to mid 70's - just a guess though. The important thing is the memories with your Dad! And, the trophy is in the eye of the beholder.