Thanks, Morgan. Breww has a tolerance for rounding issues like this, but it was designed with a Sixtel being 5.1667 gal in mind, but you’ve rounded the volume to two decimal places. (This was born out of this previous discussion if you’re interested.)
We have now tweaked the calculation of the “allowed missing volume” to be more intelligent, and you should find that with three Sixtels at 5.17 gal and a half at 15.5, you can complete the racking now without this error message.
Please let us know if you still have any trouble with this, but you should be all good!