When you change the scale on the RS meter, the reading is correct without any adjustment. So, in your case, when the meter scale is 90bd, and it reads -4db, the sound pressure is 86db. This is the same way a voltmeter works when changing scales.
RS SPL calculation when changed from 80dB
Need some help on the SPL calculations on my radio shack meter. I just measured my room response. I baselined at 80dB for 1000Hz to get 0 on the meter using the Rives CD2 calibrated test tones. When I measured the SPL of the 25 Hz tone I have to change the meter to 90 and it reads -4dB does this means it is (80dB-4db+10dB)=86db? Likewise at 12.5kHz, I changed the meter to 70dB and it reads -6dB does this mean it is (80dB-6db-10db)=64db? Just want make sure I am interpreting correctly - would like to post the data and get some responses.