Adjust it for the 25 degree list at 20C, then use the sugar calibration method to get a polynomial fomulae accurately.
You just need a container just deep enough, wide enough to allow the iSpindel to float vertically and horizontally. A 2L plastic jug worked for me, then measure the sugar in...
I am seeing exactly the same jump in SG with my four brews since building the iSpindel. Why I don't know, my temperature remains stable, I guess it just would not be noticed with a conventional hydrometer. The brew also generates a couple of degrees of heat, which I would not notice had it not...