Wilfried, thanks for the Taurus doubles list - it's quite a collection. Your "aperture needed" numbers on the far right - the sorting order of the list - are a bonus because we can evaluate the usefulness of the current formula from that.
I will say that I think we have, broadly, two types of doubles listed here, as well as some intermediate pairs and near equal pairs - and these classes will give different results with telescopes around 15cm aperture.
Class 1 is uneven pairs that have a reasonably bright companion, so typically a bright primary as well. Class 2 are dimmer, and are likely impacted by both light pollution, and by the reduced capacity of the eye to see at very low light levels. I think they're Lewis's two categories of "bright uneven pairs" and "faint uneven pairs".
So, for example, among the "bright uneven pairs" we would have STF 562, LEI4, BU 1238.
More challenging due to dim magnitudes are pairs such as BU 536 or A 2419 (possible with 18cm refractor - with smaller? haven't tried yet); CHR 213; and pairs such as A 117 where you have 9th magnitude stars closer than 1 arcsecond - I've seen some of these with an 18cm refractor, but those I've looked at with a 14cm refractor show the effect of too little light combined with being near the Dawes Limit figure for bright pairs - generally too difficult.
There are also intermediate pairs (good) such as BU 91. So there's a good gradation of difficulty.
I'm inclined to the view that the difficulty index, implied by the aperture numbers given, does not fit real world observing as well as might be hoped. For example, I'd expect STT 78 to be slightly
easier than STF 562 because of the smaller delta-m (2.67 versus 3.12) at the same separation despite the secondary star being a 1/2 magnitude fainter - unless you're observing in heavy light pollution or bright moonlight. Mag 10.5 is not too bad with around 15cm aperture - my experience in middling light pollution is that close secondary stars become noticeably more difficult at mag 11.0, quite a lot harder still at 11.5, and are very difficult at mag 12.0 with telescopes of 14 and 15cm. The drop from 10.0 to 10.5 is less significant than from 10.5 to 11.0 (etc). For smaller telescopes, 10.5 will be noticeably harder - I have a short 80mm refractor and notice the difference compared to the 140mm (3x the light-gathering is striking on fainter stars).
So I'd also expect BU 91 (despite Dm of 2.5) to be possible with 15cm; but unlikely for A 117 (9th mag components, DL separation). Based on past experience, the 18cm apo I once had the use of would show both.
One small advantage I'd mention here is slightly greater light throughput for refractors compared to some SCT/Mak systems and basic Newtonians. My 14cm refractor has about the same light throughput as the 6-inch (15cm) f/8 Newtonian I had for many years. That can help with threshhold stars. Some Maks appear to be slightly low on throughput - some recent SCTs are surprisingly bright (such as Celestron's XLT versions).
The list has a good collection of doubles. There are several on it I had overlooked and will now observe.
Re BOV 28 - that's one of quite a few doubles, particularly in Taurus, that have IR magnitudes instead of V (visual) magnitudes - the warning is the 'K' in the notes column indicating an IR-band magnitude. They're not bright in terms of V magnitudes.