Thread drift is the spice of life! 
I must have been extremely lucky or Superfil is pretty tolerant. I have used the "that looks about right" method of mixing a batch and never had one not harden or be a sanding nightmare. Just out of curiosity, since the fillet police are about to knock at my door and haul me off and keeping with the drift, how do most measure it, especially small batches. How does that compare to measuring by volume? Maybe we could use cooking measuring spoons if we knew the volume ratio.
Ken
I have a feeling, like many things in life, the answer is "it depends"... because the solids settle and separate, the mixing by weight AFTER stirring is giving you the best shot at a good final product... but what happens say if you forget to mix it one time, but you do weight both half's properly, and predictably it is no good so you chuck it and start a fresh mix, remembering to stir both half's this time?
That last batch that you didn't stir ended up taking either more solids or more hardener depending on weather you scooped up off the top or bottom of each container... so even stirred, this next batch and every batch after it is a potentially non-ideal mix ratio. I'm sure there is a lot of leeway built in and we get away with murder for the most part, but still, it makes ya think...
Oh, and you do have dedicated stirrers for part A and B right, so you don't cross contaminate? (I buy large bags of popsicle sticks).
Fun stuff, it works great. I had some that was quite old, at least 10yrs... had a rear entry wood door with some rot starting at the bottom, didn't have any Bondo handy and thought what the heck, might as well use up this old batch. Worked a treat. heh.
EricV