how accurate are travel lift strap weight gauges

Have had the opportunity to check this issue a few times.

First thing to consider that the travel lift has been recently calibrated. Then remember the straps are picking the boat up at an angle, you will have to calculate the vertical component of the recorded reading, this will require you to know the strap angle as well. Basic Vector trigonometry.

Having done this on a few different lifts, then checking on a 3D model as well as verifying the floating waterline, found a well maintained travel lift accurate to 1 - 2%.

Dry advertised weights can mean little depending on the boat manufacturer. It could have been for a prototype or a non-optioned stripped out boat. Diesel fuel at about 7.25 lbs/gal, fresh water at 8.34 lbs/gal, plus gear, options etc. and the corrected lift measurement will get you as close as you need to be.