After waiting 114 seconds for SED to load my 800 KB script in 6.40.5 (to test this, and possibly acquire a screenshot), I couldn't figure out how to watch a variable through setting it in SED. I never do it that way (and I never knew SED even had this feature).
I "watch" variables by using a debug panel that, with a press of a single key (not the F11 debug panel, since it's unedittable, but rather a self-made one), is turned on and off as a toggle. I just add the variable as a "watch" in this debug panel and monitor it that way.