You subtract the lowest from the highest RI to find the birefringence value. In the Quiz example, from those high and low RI values at 6 positions, the highest RI value is 1.694, at position 5. The lowest is 1.671, at position 1. You don’t average these, you just record them. (If you were testing an actual gem, you would check both the high RI and low RI for maximum readings as discussed here). 1.694 - 1.671 = 0.023.
So, the RI value range for this gem is 1.671 - 1.694. Its birefringence is 0.023.
I hope this helps.