Logic would push me to say measure from your factory settings, eg that might be 1deg of toe at ride height. It certainly wouldn't hurt to measure both though and get the measurement at 0deg toe also. Just remember the toe in on suspension compression will be amplified the more toe there is at ride height.
Bacause this is an exponential curve the difference between 1 and 0 toe at ride height will be the position of the curve, eg it gets shifted left the more ride height toe exists. As part of this, the point at where a certain degree of toe is reached is achieved alot faster the more toe you start with.
I just found a few links that appear to have formulas for this already. Have a look;
http://www.nissanroadracing.com/showthread.php?t=1445
http://www.speed-wiz.com/graphs/steering-graph.htm
http://www.speed-wiz.com/calculation...simulation.htm
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