The NH NAD83 projections use a central ("Longitude of natural origin") at -71.6666666666667. This places the central meridian at the center of NH.

Convergence angle are ~ 
0.62W at W edge, < 0.09W in Mason, ~ 0 in Brookline, 0.67E at E edge of NH

Given that, the convergence angle along the NI/Mason border is less than 0.1 degrees; so Grid North is very close to True North.


42º43'15.31747''
-71º48'28.04219''
Grid N = True N at long of origin, = -71.66666 for UTM zone 19
At our W boundary, True N is 0.09ºW E of grid N, 0.05ºW in East Mason
Azimuth tool reports True N

4733647.628
270101.209

42º42'52.05928''
-71º48'23.18278''

4732926.477
270187.88


4733647.628-4732926.477
270101.209-270187.88

https://www.fcc.gov/media/radio/distance-and-azimuths
   Distance between:
    42 43 15.32 N Latitude,    71 48 28.04 W Longitude (Point 1)
     As decimals: 42.7209222 Latitude, -71.8077889 Longitude

   and

   42 42 52.06 N Latitude,    71 48 23.18 W Longitude (Point 2)
     As decimals: 42.7144611 Latitude, -71.8064389 Longitude

   Distance = 0.727 km (0.452 miles)
   via the Great Circle method of computation

     Azimuth, Point 1 to Point 2: 171.27° True
     Azimuth, Point 2 to Point 1: 351.27° True

i.e., N 8.73 W true
Declination 1991-07	15.46	  {CCW from N}

so N 6.73 E magnetic

---------------------
https://gis.stackexchange.com/questions/115531/calculating-grid-convergence-true-north-to-grid-north 2

I've always used this formula:

CA = (λ - λCM) × sin φ

Given: CA is the grid Convergence Angle for any transverse Mercator projection; 
 λCM is the longitude of the zone's Central Meridian; 
 φ, λ are the latitude and longitude of the point in question.

Pay attention to the resulting algebraic sign!



Complementing the answers by @Martin F and @V. Kelly Bellis, an article by Ontario land surveyor Tim Hartley (see p.4) gives the formula:

CA = 32.39 arc seconds * (distance from central meridian in km) * tan(latitude)
The other answers require longitude of each point where being calculated,
while this formula can be used with the UTM/MTM X coordinate, yielding:

CA = (8.997E-06) degrees * (delta X from central meridian in m) * tan(latitude)

You still have to calculate tan(latitude), but in tropical and temperate
regions tan(latitude) changes much, much less per delta Y than the linear
dependency on delta X, (and the convergence error of true longitude vs delta X
itself) so you can do this once for your whole map for a survey area.

I expect this is equivalent to @V. Kelly Bellis's formula, with the tan()
representing the 1/cos() dependency of (spherical) ground scale by latitude
combined with the sin() in that formula, with units converted.

-----------------
https://gis.stackexchange.com/questions/115531/calculating-grid-convergence-true-north-to-grid-north
UTM 19, 66º to 72º, central meridian = 69º


for our parcel's W border, Lat = 42.7, Long = -71.8, 
CA = (8.997E-06) degrees * (∆ in METERS ) * tan(42.7)
	= 8.997E-06 * ( ??? ) * 0.92277  = 1.11 e-6

-----------------
https://www.maxgeo.com/which-way-is-north-really/

-----------------

NAD83(NSRS2007) / New Hampshire (ftUS)
Properties
Units: feet
Static (relies on a datum which is plate-fixed)
Method: Transverse Mercator
WKT
PROJCRS["NAD83(NSRS2007) / New Hampshire (ftUS)",
    BASEGEOGCRS["NAD83(NSRS2007)",
        DATUM["NAD83 (National Spatial Reference System 2007)",
            ELLIPSOID["GRS 1980",6378137,298.257222101,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["EPSG",4759]],
    CONVERSION["SPCS83 New Hampshire zone (US Survey feet)",
        METHOD["Transverse Mercator",
            ID["EPSG",9807]],
        PARAMETER["Latitude of natural origin",42.5,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8801]],
        PARAMETER["Longitude of natural origin",-71.6666666666667,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["Scale factor at natural origin",0.999966667,
            SCALEUNIT["unity",1],
            ID["EPSG",8805]],
        PARAMETER["False easting",984250,
            LENGTHUNIT["US survey foot",0.304800609601219],
            ID["EPSG",8806]],
        PARAMETER["False northing",0,
            LENGTHUNIT["US survey foot",0.304800609601219],
            ID["EPSG",8807]]],
    CS[Cartesian,2],
        AXIS["easting (X)",east,
            ORDER[1],
            LENGTHUNIT["US survey foot",0.304800609601219]],
        AXIS["northing (Y)",north,
            ORDER[2],
            LENGTHUNIT["US survey foot",0.304800609601219]],
    USAGE[
        SCOPE["unknown"],
        AREA["USA - New Hampshire"],
        BBOX[42.69,-72.56,45.31,-70.63]],
    ID["EPSG",3614]]
Proj4
+proj=tmerc +lat_0=42.5 +lon_0=-71.6666666666667 +k=0.999966667 +x_0=300000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs
Extent
-72.56, 42.69, -70.63, 45.31