Hi Martin,
I just mean that you are only measuring your response. And then we're just talking about it and analyzing it as humans. That target curve is where you want to end up after the BFD is done filtering, and will be used to determine such filters. When it comes time to set actually set filters, that target can be manipulated by you to get various results (you need a BFD to get anything actually done, but you can calculate the filters in REW before having the BFD, if you like). Since you're just measuring to see "where you are", the target is irrelevant and even could be removed from the graph. I think that's what's going on, anyway.
If you read through all the REW instructions, it goes through all the steps to get the system calibrated. Usually, we go at a target of about 75 dB. Once you get everything calibrated, there's a button called "Set Target Level". This will set that overall target based on the output from the sub before EQ. It usually ends up in the mid-70s. You can move it around if you want, but REW does a pretty good job of getting everything right on its own.
As to the cal file for the SPL meter, I'm not quite sure if it's right. If the mostly-flat black line in your picture is your SPL meter cal, I don't think it's right. You would know if you loaded the cal file. It's something that's available
here on HTS. Find your meter and load it up under the "File" menu in REW. My apologies if you already did this, but I'm pretty sure there's more of a dip at the low end for the RS SPL meters than your black line shows. Actually, looking back again, I think you may have done the cal for the soundcard. The SPL meter cal is separate.
Unless your sub's volume level is high already, I would probably turn it up before doing the real REW mesaurement. If you get spikes like those in your mains, that's OK. We're going to tame those with the BFD. Plus, your sub level is quite a bit lower than your mains. How does it sound now from your point of view?