This gentleman has already explained it.I have a couple of these meters also and use the battery adapters. First, the scale read by the pointer is not in EV; you must read the pointer scale, then rotate the dial to the pointer indicated number on the yellow scale (yellow triangle) and then read the EV below that with the black triangle.
A quick check of accuracy: set ISO to 100, mode to incident reading (slide dome over center), aim at sun on bright sunny day. Reading should be 20 (plus/minus a smidge) on the pointer scale, set 20 on the yellow scale and EV should read 15. If this is so, all is well and the meter is accurate.
BTW, Gossen meters indicate about 2/3 stop higher than most Japanese calibrated meters. German meters use a different calibration protocol than Japan; German meters use 5600K vs. Japan's 3400K color temperature light sources (might be off a tad on the temps as I am recalling these numbers from memory).
The usual problem is that the protector/lens on the sensor gets cloudy from grunge/oils and the meter will read lower than normal. The meter has to be taken apart and the lens and protective cover need to be cleaned.
The Luna Pro is a great meter. The only drawback is that the meter has a bit of a 'memory;' if you have read a bright area and immediately read a much darker area, the meter needs several seconds to settle into the correct reading. This is inherent with all CdS sensors.