Determine temp difference setting for up/downstairs?

We have two-zones, one for upstairs and one for downstairs. The upstairs is not in use, so we would like to keep the temperature up there lower and save on oil costs. The rub: our great room has open, vaulted ceilings up to (actually past) the second floor, so lots of heat will "escape" to the second floor.
My guess is that there is a point of diminishing returns where, if I have the temps set too differently, so much hot air will head upstairs that the downstairs zone will be perpetually on, and we will lose any potential savings.
Is there a metric of some sort to help figure out how different we can have the two temp settings? If we would like to keep the downstairs at a constant 68 degrees when we are home, how should we figure out how low we can "afford" to keep the upstairs?
Thanks much for any help.
jtf
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