Hi Graham,
Others may disagree, but if I were you, looking to maintain a sagitta of around 1.2mm, then skip the #120 grit and go with the #220 next. 1 hour to 90mins should be all you need to remove all pits left from #80.
I would use a little TOT during this time to:
a) remove more quickly pits (#80) from the edge regions of the mirror, and
b) to control the depth of the sagitta.
The smaller the number of grades of grits used the less likely large grit contamination is to occur as one goes through the fine grinding process.
I would suggest the following progression of grits for your 6" mirror:
#80, #220, #400 (1 -2 hours last two), #600, #1000 (1 hour each).
Use a little TOT with each grit to get a uniform across the surface grind and to manage sagitta. (though sagitta will be less sensitive to change as the grit sizes get smaller).