I have been a user of ArcGIS Desktop for many years and this function is easily done within ArcMap using the 'Calculate Grid Convergence Angle (Cartography)' tool.
I am wondering if there is a similar function in QGIS.
A little explanation of my process.
I deal with directional surveys for wells on a constant basis. Typically these are reported to the state in their local State Plane coordinate system and as Grid North.
Since i work in a project that contains multiple states however, converting them to True North instead of their typical Grid North (say New Mexico East NAD83 and Texas Central NAD27) is how i keep them accurate in a NAD27 BLM 13N project. The geologics software I use does the proper rotation for them as long as they are true north.
What I have typically done is create a fishnet in ArcGIS and kept the point file that is a byproduct of that.
In that point file i create fields to store the X,Y, and convergence angle for each point, for their local CRS. I then translate each local CRS (keeping the convergence angle of the original CRS) into the combined NAD27 BLM 13N project and can make a grid out of those points for each states wells. I can then sample the wells to a grid and subtract the convergence angle from the azimuth of the wells directional surveys to create True North surveys.
In QGIS I have found how to do the fishnet portion (much easier) however I cant seem to find the same functionality for storing the convergence angle in a field of that shapefile. I have looked in the field calculator under geometry and typing in various searches. I have discovered how to calculate the X and Ys just cant find that darn angle. For NME NAD83 SPCS convergence angles should be around -.7 to +.6 degrees and Texas Central (since Texas went horizontal with its SPCS's) is around -3.5 in the East to +3.25 or so in the West.