Not sure, but I would have to assume it's a linear relationship....Was looking at the Bosch GPL5C as the local supply shop has them for a fairly decent sale price
Specswise : it reads Up to ±1/4-in @ 100-ft
Not sure how it competes at 30 feet, with my PLS 4 rated at 1/8 at 30 feet.
1/8 at 30 = 3/8 at 90 = 3/8+ at 100 or
alternatively 1/4 at 100 = approx .08 at 30