have you tried interpolating? i believe thats what they expect you to do
or if its a beta, multiply the OAT by 3, put the decimal point in front then add to 24 and deduct (i think) 0.1 for every 500ft indicated with 1013 set e.g.
15c = 24.45 less correction for pressure, should be same as graph (beta only)
regards
CF