Sanitary Sewer Manhole Invert Elevations

We've all made many measurements of these over the years. Our firm is going to be measuring a very large number of these in the near future, and we are wondering if anyone has a technique to accurately measure the depth of drop manholes, where a pipe enters the side of the manhole above the invert flowline. Something more accurate than reading a severely slanting rod and guessing how many hundredths or tenths to shave off the "slope" distance. Perhaps some type of a hinged "T-square" device that can be lowered into the manhole, opened, placed into the pipe, a measurement made, and then collapsed and removed? Any tips, hints or tricks would be helpful. We are NOT entering ANY confined spaces, so this has to be done from the surface. Thanks!