Determining the step-down ring sizes to step down a camera lens

[Jannis]

I will receive my new QHYCFW2-M filter wheel with a set of Baader HaLRGB filters any day now, and i will most likely be starting my imaging season with camera lenses and the Polemaster as my main imaging camera.

My two main lenses will likely be Canon 50mm F/1.4 and Olympus OM 135mm F/3.5. I might also be using an EF-S 55-250mm f/4-5.6, but i will have to see how it perform first...

Naturally the most easy way is just to select a lower F step on the lens, but this causes unwanted star spikes (as well as it's not a manual setting on the Canon lenses, so changing it is not too easy). To prevent this i've seen some people use simple filter step-down rings to manually step down the lenses without getting any artifacts.
This seems like a very simple and good solution to the problem as i will how have the filters behind the lens instead of in the front, however i'm not sure how to calculate what step-down ring size i will need to reach the different F ratios? Is it as simple as measuring the size of the opening of the step-down ring and the distance? Like, say i get a 58mm-25mm step down ring and attach this ot the 50mm lens. Will this then give me 50mm / 25mm = F/2 (roughly)?
Also, is this a solution that causes any other known problems?