The water damage classes I took always recommended taking your first readings when you set up your equipment, not before. The purpose of taking dry log readings is to show your equipment is working properly and your system is balanced, your evaporation is not exceeding your dehumidification.
I will let my air movers and dehumidifiers run for a bit before I take my initial readings to get some moisture in the air and give the coils in the dehumidifier a chance to cool down. This will give a more accurate documentation of grain depression. I also like to take affected surface temperature readings to show vapor pressure differentials between the ambient air and wet surface. I am not sure if this answers what you are asking.