I Surface Differential Rotation by measuring the position of sunspots

[TheSpectacularSpecs]

TL;DR

I am measuring positions of sunspots and use them to calculate the rotation of the Sun. Wikipedia has an equation that is very simple for surface rotations at different latitudes, but I have not been able to find the equation properly sourced.

I am doing a University lab project where I measure positions of sunspots (using images from NASA's SDO) and use them to calculate the rotation of the Sun. Currently, all is going well: I have the angular velocity of several sunspots at varying heights. However, I want to be able to find the angular velocity at a specific latitude, especially the equator. Wikipedia has an equation that is very simple for surface rotations at different latitudes, but I have not been able to find the equation anywhere else, and Wikipedia does not have sources for that particular equation. Wikipedia, although significantly better than it used to be, is not an acceptable source to reference in my project. Does anyone know if this equation is true and where I can find a referenceable source with it?
Equation: https://ibb.co/7Ys3WHG (or on https://en.wikipedia.org/wiki/Differential_rotation under the Surface differential rotation heading)

 

[TheSpectacularSpecs]

For anyone wondering, I have confirmed this equation is at least relatively true, with:
ω(Φ) = A + Bsin²Φ
where A is the equatorial rotation and B is the latitudinal gradient of the rotation. This is all the information that I need, and was found in the article cited below.
K. J. Li, X. J. Shi, J. L. Xie, P. X. Gao, H. F. Liang, L. S. Zhan, W. Feng, Solar-cycle-related variation of solar differential rotation, Monthly Notices of the Royal Astronomical Society, Volume 433, Issue 1, 21 July 2013, Pages 521–527, https://doi.org/10.1093/mnras/stt744