First you want to have some baseline measurements taken during periods when you feel "normal" -- for potential hypotension, hemoconcentration, hypoalbuminemia and the presence of the MGUS.
And then you'd like to have those same measurements taken (ideally, more than once a day) whenever you feel "abnormal," namely, after you begin to swell and all the way up as the swelling reaches its peak.
Swelling in SCLS occurs because (a) plasma that normally resides inside the blood vessels starts to leak out into body cavities (usually, the extremities), and (b) in the wake of the resulting shock, physicians inject fluids into the veins of SCLS patients to resuscitate them, but this fluid is also leaked out, worsening the swelling (among other consequences).
Once the episode passes (say, 2-4 days), and if the patient has survived, the leakage ceases, the loose plasma is reabsorbed, and patients return to their usual weight.