I have spent a bit of time trying to resolve the ambiguity of 16 locations of interupting letters and I may have come up with a solution.
Firstly I assumed that “interupting letters” in the penultimate sentence of the preamble referred to all instances of the particular letter that does the interupting, not just those instances that actually interrupted a ring entry - so the centre letter can be counted.
I think you have to assume that “location” refers to a specific place on the page so it is not logical to count 2 or more uses of the same location as separate locations.
I think “16 in all” means 16 separate locations of the letter rather than 16 scores. Indeed I have 17 scoring entries.
I have 12 instances of the letter in the rings and one in the centre. I also have three radials with no use of the letter (7, 8 and 20). I assume these shots missed the target and scored zero. So all I needed to do was to write the letter in three random locations outside the target (outside the outer ring) and I now have 12 + 1 + 3 = 16 locations.
A bit convoluted but the only way I can make it work.
Or 16 might be a typo.......