The minimal variance is 0, and you get the maximal variance if 50% check the minimum and 50% check the maximum. With a scale from a to b, the variance is then given by approximately 1/4(a-b)^2.
You can convert this to a score just by multiplying the variance with 400/(a-b)^2. I would expect that most of the values are below 50. It would be possible to modify the mapping variance -> score to get more values larger than 50 (without ruining the order), if necessary.