Are you shooting the top id and then subtracting the id to get flowline? Will the JMate shoot through the liquid or give a reading to the surface of the liquid? The handheld laser distance measurers that I have used wont shoot through the liquid.
Curiosity got the better of me so I performed a quick "bucket" test.
Here are the results starting with an empty bucket. Water depths listed in decimal feet. As you see it is shooting thru the water but the measurement is distorted slightly as the bucket fills up. I would say for your normal sewer lines with .1 to or .2 of depth it is okay to use the readings. Anything over that I may adjust the measurement based on what I've found below. It's fairly linear until the jump from .5 to .8. Now I am curious about deeper water. Never did it give a return to the top of the water.
C31,-1.011,-0.052,-6.287,Empty bucket
C32,-1.011,-0.052,-6.287,Empty bucket
C33,-1.015,-0.050,-6.314,.1
C34,-1.019,-0.048,-6.341,.2
C35,-1.024,-0.046,-6.376,.3
C36,-1.028,-0.045,-6.405,.4
C37,-1.034,-0.043,-6.440,.5
C38,-1.033,-0.043,-6.431,.8 Full bucket
C39,-1.011,-0.051,-6.288,Empty bucket