From Discworld MUD Wiki
A few (non-empirically tested) observations:
Chance to shoplift may be around 50% for items valued at .15 x <co.ma.st bonus> in AM$, modified by shop security.
Increasing co.ma.sl from bonus 228 to 276 appeared to markedly slow down the rate at which difficulty of repeated shoplifting increased when stealing while hidden. Hypothesis: a store's "personal security rating" from repeated shoplifting is reduced or eliminated with high enough bonuses in certain other covert skills besides co.ma.st. Possibilities include: co.st.in (.ou for outdoor shops, .un for theoretical underwater shops), both co.hi.pe and .ob, co.ma.pas, and co.ma.sl