In the 707perf.zip you have the 707Performance2.pdf. Go to page 13 and, right at the top of the page, you can find a chart that gives you the Optimum Altitudes at a given weight and Nautical Miles per 1000lbs of fuel at a cruising speed of M.80

I personally tend to fly at between M.82 and M.84. I find that the throttles at about 60% (according to the mouse over Tool Tip on the throttle levers) she sits close to M.84. This lovely lady loves speed! ;D

Hope this helps

Mark