Just a little calculations on the acceleration for a solar Sail:
The impulse p of a photon of frequency ν, thus of energy hν , thus of wavelength λ = c/ν on the sail is p = hν/c = h/λ
where h is Planck's constant (6.6 10^-34 Joules). To simplify the calculations, assume that the sun does not emit a whole rainbow of colors, but only the yellow-green color (λ = 500 nm) . We have then p = 1.32 10^-27 kg.m.s-1 and the energy Ep = hν of a photon will be 3.957 10^-19 J
Our sun has a total power Es of 3.9x10^26 W. It emits np = Es/Ep = 9.85 10^44 photons per second. But the Earth is 149 million kilometers from the sun. All these photons are distributed on a sphere of the same radius R, whose surface is S = 4πR2 or 2.79 10^23 m2. This gives us a density d = np/S = 3.53 10^21 photons per square meter per second.
Quite a lot! But each of these photons carries a tiny pulse : (the p above). The impulse received by a square meter of solar sail will therefore be i = p.d = 4.66 10-6 kg.m-1.s-2 In other words, a sail of 1m2 weighing 4.66 milligrams will accelerate at 1 m.s-2, that is to say about one tenth of a g. If we want to accelerate a mass of 1 kg with this same acceleration, the surface of the sail will have to be 1/i = 214 456 m2, a square of 463 m of side (and thus weighing 1 kg!). This requires extremely light materials, but it is not utopian.
Note : if the craft is a a distance D (in astronomical units) from the sun, just multiply the above acceleration with 1/D^2
However there is an easier way.
Radiation pressure from reflection is just $$P = 2\frac{I}{c}$$ Newtons per square meter where $I$ is the intensity of the light in watts/m^2 and $c$ is the speed of light. At 1 AU $I$ is the solar constant of about 1361 W/m^2. If we do it this way we don't need to worry about approximating with a single wavelength.
To conclude and answer the question, if you have the radiation Pressure (in Newtons), you divide it by the craft mass (in kg) to get acceleration in m.s-2(Newton's F=mA). Once you have the craft acceleration, just integrate over time to get the velocity increase. Obviously, as the craft distance from the sun may change, you will need to do it in (likely thousands of) steps with a computer.