Plastic milk jugs are too inconsistent.
If you want to test penetration and expansion, the best thing is a Fackler Trough, a trough loaded with plastic baggies of water.
On the cheap, old fashioned square waxed cardboard milk cartons are better than plastic.
The conversion is 1.8" of water in bags or 1.5" in cartons = 1" gelatin.
So 15" of gelatin penetration would require 22-27" of water travel.