The best way IMO is to get a printout with your alignment specs including the thrust angle. Shimming the rear will move the thrust angle, but will not adjust rear toe.
It really isn't something easily measured and it is mush easier to correct on an alignment rack. Loosen your front spring hanger and install shim between hanger and vehicle frame. When dealing with shims a little goes a long way. We shim rear ends on fwd solid rear axle toyota cars all the time here.
You will need to find a friendly alignment shop or a friendly alignment tech that doesn't mind a time consuming after/off hours alignment. Most shops don't like to tie up a rack during business hours with old cars and alignments that are more than toe N go.