First, since you have a Ca car you should have 85% KOEO (key on, engine off). This corresponds to the inverse of your 15% reading and is thus probably correct except for convention.
Since your convention is off, the desire to set 45% will in effect give you 55%. If you set it to 50% it won't matter. Using your convention setting 55% will get 45% true to convention.
To understand convention one must understand duty cycle. If you look at the signal with a scope you will see in one period a high signal and a low signal. The relative amount of each is the duty cycle of the period. So the statement is either: they duty cycle was high 85% of the time or it is low 15% of the time depending on your point of view. Since the proper convention is to state that the condition that exists at KOEO is 85 on CA cars you must change the reference on the meter or do the inverse calculation.
Bosch Master, ASE Master, L1
33 years MB technician