In addition to the layout plans on the Mass DOT website, there usually are field books in the Distict offices, generally filed by town/city. If the right of way is old, say before 1925-30, the monuments are generallt granite with the center of the back edge being the mark. The baseline measurememnts from early layouts are usually not too great, but in the '30s a lot of them were remeasured. Only the field books show the newer measurements, not the layout plans. Also, the layout plan baselines were never changed to reflect later remeasurements.
I traverse and locate as many of the monuments as can be recovered, both sides of the roadway. Then I fit a trial baseline between the monuments and tweak where necessary to get a best fit, ignoring stationing but closely splitting the monuments. This generally reveals positional errors of monuments that have moved, and gives reasonable positions for those missing. While the monuments represent the right of way lines, the takings are generally of specific width like 60.00', 150.00' etc. In most cases you will find the monuments to fit closely to the baseline offsets. Once I get a best fit baseline, I apply the offsets to re-establish the right of way lines as property lines.
As an example, I traversed about three miles of Rte. 138, laid out in the teens. About 50% of the original monuments were missing (I dug pits, some deeper than six feet, to make sure they were missing). Most of the ones I found were very close to the offsets called (60 foot layout), usually within less than a tenth. I checked stationing starting at the beginning of the layout (Town Line) and it varied closely proportional to the distance along the layout. Over the three miles it was off a total of about 3.50 feet, but the monuments closely fit when the proportioning was applied.
Newer layouts are usually very close to the plan stationing if you can find the monuments.