hm, formula doesn't take into account resonant frequency and wind speed?
Interesting read and the technical refinements are interesting, just not too practical for Jo Public. There are many "Rules" in photography and too many people take them as LAW. They are all really general guidelines its up to the photographer to try them as a starting point if in a hurry and they will generally yield an acceptable shot (so you at least get something) but you then refine settings from there to get the shot you are after for the result you want. Understanding is often missing.
Streaky/blobby stars are not just a factor of exposure time, lens distortion/cleanliness, tripod sturdiness, internal/external vibrations also can contribute. I usually start with a test shot at the 500 rule exposure time.Then look at the shot on camera making sure my target is in shot (and should remain in shot for my session (if i'm not tracking) then pixel peep for streaking, if everything is pin sharp or slightly streaky in the middle of my target then I readjust exposure time one increment in either direction then take another test shot. Rinse and repeat until I find the longest exposure time to keep round stars, that's the time I use for that target, that session.
If lens distortion is noticeable I adjust aperture again testing to find optimal for what I want. Same with ISO to get cleanest, optimal exposure for sky conditions.
Moving parts such as shutter and mirror inside cameras can create vibrations which can limit the roundness of stars too. So understand your camera, some have features like mirror up to reduce this. Likewise wind, people moving around, nearby vehicles can all transmit vibrations that can result in star distortion in shots. If shooting on camera tripod, don't extend it fully if you don't have too, to make it more sturdy. Good tripods often have a hook or hanging point at the base of the central pillar so if you hang a bag or something heavy from it it can help dampen external vibrations.
Only takes a little time to get this sorted up front. If you're waiting for astronomical dark time to collect your shot set, then go through all this ahead of time during the twilight while you are waiting. At least you can get focus , shutter speed and tripod vibrations sorted, then do the rest for exposure at the right time. Don't forget to set up your intervalometer. Now you are set to take a set of shots you can start to process.
As far as I'm concerned the only rule you need to follow to get a good photograph is Understand How to take a Good photograph and Why you need to adjust settings. There is no magic button, You have to Learn so deal with it.
So, the 500 rule is just one starting factor for taking non-streaky stars shots but it gets you in the right ballpark, but you still need to find the optimal settings for the session and target if you want to get picky. This applies to any budget, you do NOT need expensive cameras or lenses and the rules change depending on What you are shooting: if you just want nice pretty stars in night landscape it will differ from shooting the moon which will differ from shooting the Orion Nebula and will differ from shooting a constellation. Basically it all depends. And it will change if you are doing photometry or want an artistic shot or just want your first shot. The Rules are are generally good starting points (Note the word: generally does NOT mean always) but from there you need to refine and learn.