Darth,

I'm not sure if I am understanding your question Are you using the analog RS SPL meter, and you are you are running through a frequency sweep?

Generally speaking, you want to choose the trim setting that will give you the best frequency response. So the setting that stays close to 75dB would be desirable.

However, keep in mind that if you want to me more exact, you will have to use some correction factors. At lower frequencies, the meter understimates SPL.