I've long used calibration target media as pre-capture references in my workflow and this generally is the accepted norm in institutions that perform bulk media digitization. Doing so periodically is a good way for me to detect whether my capture hardware is degrading or if my workflow changed in some way that I missed--be it user error or SW updates. Examples for me are professionally mastered calibrated cassette and reel to reel audio tapes (the calibration run verifies speed, W&F and frequency response) and reference film slides to verify effective resolution and DR. Over the years it has definitely helped me detect failing hardware or some mistake I made in my capture setup that I might otherwise have missed.
I've really struggled with a methodology I'm satisfied with for magnetic video media like VHS and SVHS though. I'm not aware of any reference or calibration tapes - bench calibration is typically handled via signal generators feeding signals, which I suppose makes sense given the frequencies involved? But it does leave me without options. What I've done the last few years is use a couple of tapes I created myself... fresh off receiving a pair of AG-1980P players serviced by Tom Grant, I recorded 10 minutes of video (Snell & Wilcox targets, short clips from movies, clips I recorded myself) all from 1080p/4k down to s-video using the absolute best converter I could get my hands on. I recorded two identical SVHS tapes, one with each deck, then captured each tape with each recorder (4 captures) and compared them and couldn't really see a difference. This gave me some comfort that I have a couple of identical tapes made on two recorders that if not perfect are at least consistent. I typically recapture from one of these tapes every few months or when I'm using a deck I haven't used in a long time.
I suppose I'm happy with this method--the recording really does look good, much better than anything I've seen on VHS/SVHS--but I'm not sure if it really could be considered a 'reference' recording by any means. And as far as I can tell I have no way to know, which bothers me a little bit. So I was wondering if anyone else has thought about this and what if anything you've done.
By the way, the only other source I have are pre-recorded SVHS tapes--I've bought a few over the years thinking they might provide a better source of resolution, definition, etc. but my fresh recordings to new tape consistently looks better than the old pre-recorded tapes so they're just sitting.