I used a Calc-u-acre on a 1460 CIH for several years, it had a sensor I mounted above the throat, and a magnet on the throat.  When I raised the head at the end, it shut off the acre calculation.  Only problem was you had to raise the head all the way up, or at least far enough the magnet could be "read" by the sensor.

I was very happy with this simple and cheap monitor, if you weighed and calibrated properly, it was very accurate in acres and yield.

My FIL and I figured our splits based on the readings, then when all the corn and beans were sold, we compared our bushels sold with the bushels indicated on the Cal-U-Acre.  We then adjusted the bushels for both of us based on the percent of error.  We were almost always within two percent error, several times were close to perfect.  We also learned that the combine must be level when unloading, if on a hillside there were lots of error due to the sensor not opening and closing properly.

Most of our error was when the moisture changed a lot in corn, as when one variety of corn didn't dry as well as another variety. 

My unit was changed by the dealer when I traded for a CIH 1680.  They didn't know what they were doing, and mounted the sensor incorrectly.  It never did work well after that.