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.