Spreadsheet showing deflection of bolts off plate armor with uniform thickness 20 at varying values of IMPACT_FRACTURE. Where indicated, IMPACT_YIELD is set to zero, and otherwise vanilla values are used for all parameters. I also used Arena Dwaves (see thread).

Update: I added column headings to each of the data sets within the file. They are:
Force - value of SHOOT_FORCE used.
deflection - % of bolts deflected
nonserious - % of bolts either deflected or causing only bruises
tears - % of bolts causing "tearing"
bruises - % of bolts causing bruises
chips - % of bolts chipping bones
fracture - % of bolt fracturing bones
jams - % of bolts causing jams
bruises only - % of bolts causing ONLY bruises, and no other injury

Update #2 - I found that the sigmoidal transition seems to fit well to
% nonserious = 1-1/(1+exp(-(F-F0)/(F0/50))) for F<F0 and 1-1/(1+exp(-(F-F0)/(F0/25))) for F>F0.
This is a logisitic function, different widths above and below the cutoff force.