If your receiver volume display says 70 and your sound level meter (SLM) reads 70 dB(SPL), it's purely coincidence...unless you initially set it that way using the trims on each channel.
However, if the volume display increments in dB, then you should notice that the sound level display on your SLM correlates with the receiver volume display as long as you don't move the microphone. If you do move it to another spot in the room, 70 on the volume display might correspond to, say, 72 dB(SPL) on the SLM. But as long as the volume display is in dB, you will still see a 1:1 change on the SLM. I.e., turn the volume up to 72 and the SLM should now show 74 dB(SPL). There are other factors/variables to consider that I am ignoring right now (e.g., relative contribution of reflected energy), but generally this should be the case.
In order to "calibrate" your receiver, you should already sort of be there. If you have 70 = 70 for a certain microphone location, then there should be 1:1 correlation of any other volume settings, provided the receiver volume display increments in dB.
FWIW, I would not call this "calibration" so as not to confuse it with the actual calibration of an SLM. I would call this "correlation"; correlating the volume display reading with an SLM reading. Once again, provided the volume display increments in dB, there should, in general, be correlation. E.g., some receiver volume displays top out at 0 dB, with negative values below that. What one decides to use as the starting point in that case is arbitrary. One could match -25 dB volume display to 70 dB(SPL) and have 1:1 correlation from that point.
Make sense?