Whats a good freq to measure baseline SPL when calibrating a sub?

So I finally got my REL T/5i sub and will be calibrating it this weekend.  I have an SPL meter and a test tones CD.  I know the first step is to pick a test tone to set the baseline SPL from my monitors which I will use to compare the  low freq response of the sub.  Can someone please recommend a good starting freq?