If I understand you question correctly: you are asking if you use the SPL meter to set the relative level in REW before doing your sweeps?
Then yes, you set up the test rig to have your microphone ready to record the sweeps. Before doing that, you set the level through the options window. One of the option is to play full range or sub level pink noise and then put what volume you are recording. Simply hold the SPL meter next to the mic position, record the reading, and put those results in the dialog box that pops up during the calibration.
Aim for 70 to 80 dB. Remember this is just setting a relative level so that when you see a measurement showing a spike to 90dB at a certain frequency, you know that is accurate (and semi-repeatable if you come back later and take more measurements).