Look at your C-tier results and figure out the SSA by looking for the score nearest 1000 rating. If a 49 is 1005 and 50 is 995, then the SSA is 49.5. If you didn't have anyone shooting a round rating near or over 1000, then you have a little more math to do. If a 53 is 965 and a 56 is 935, then there are 10 points per throw (30 pts/3 throws). 35 points added to 965 is 3.5 throws less than the 53 so you end up with the same SSA of 49.5. This works for any tournament results to determine SSA for a round.
Rater of the tossed arc.