The formula for the radius of an Einstein ring is

(x gravitational constant x mass of lensing object / speed of light squared x between observer and lensed object / (distance between observer and lensing object x distance between lensing object and lensed object))

where the distances are "angular diameter distances" measured in terms of the object's actual size divided by its angular diameter. Given the radius of the Einstein ring and the mass of the lensing object, the *relative* distances of the quasar and its lens can be calculated, but the actual distance to one is needed to know the distance to the other.

Usually, though, the distances to the lens and the background object are determined by measuring their redshifts.
Source(s):
http://en.wikipedia.org/wiki/Einstein_ring
http://en.wikipedia.org/wiki/Angular_diameter_distance

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