Nice video Doc; same method I was taught when doing my apprenticeship 50 years ago...
+1 ... 'I use aluminium welding wire for the rocker support, its squashes into the uneven services and flattens on the loading jaw'.
Same here...but above all ensure that your vice is sitting flat & square to your quill otherwise you'll not get any square sides. When I got my vice I ran a dti down my vice jaws & found about .004" discrepancy top to bottom, had to clean up the jaw seats, then ran dti along a 1-2-3- block set up in vice on parallels, front to back ( 'Y' axis ) & found a discrepancy of .007" ended up machining .004" off base, which revealed a hollow in the centre of the base casting... casting stresses
, the .004" off left me with the casting sitting on the outer edges, but flat all round, so it's always advisable to check vices first.