The instruction to set the volume to a given level and then adjust the channel trims until the spl meter reads 75 dB or 85 dB or whatever you choose is only applicable to receivers that do not automatically set the level of the master volume control. Some receivers (mostly THX certified receivers) automatically set the level and it doesn't matter what the volume display reads. Either way you have to adjust the channel trims to get the spl reading you want.
Here's an example: My Onkyo 502 doesn't automatically set the master volume level. I just happened to choose 60 (it goes 0-79) for my reference position. If I want Dolby Reference Level and were using the AVIA disc (test tones are at -20 dB), I would adjust the channel trims until each channel reads 85 dB. I could achieve the same thing by setting the volume to 50 and adjusting the channel levels until the meter reads 75 dB. That would still put reference level at 60 on the volume display, but 75 dB tones are easier on the ears than 85 dB tones. Note also that the typical recevier has a range of -12 dB to +12 dB for the channel levels. You also have room to play there. In my old apt with a much smaller living room, I could get the same reference level at 50 by having the channel trims set nearer to +12 or have it at 60 with the channel trims set to around +2. In my house it doesn't work that way because the room is much larger and there isn't enough range on the channel trims to get reference level at a lower volume number on the volume display.
On a receiver like the 702, the master volume is set automatically to 0 dB, so you don't have a choice - 0 dB on the volume display is the reference volume position.
You can choose to follow Dolby Reference Level or whatever you like. The key thing is that the level of each channel is equal. I think you'll find that you rarely listen at true reference level anyway - it is very loud. 60 on my receiver yields reference level, but I rarely ever have it louder than 40.
