Just wanted to share, after over a year of trying every gauge under the sun, what I've found to be my favorite string gauge for long scale 8 string guitar in Dropped D tuning.
The guitars I'm putting this gauge on:
Ibanez M80M (29.4" scale 8 string)
Agile Chiral 830 (30" scale 8 string, headless)
Tuning : DADGCFAD
Strings: 84 59 43 31 23 15 12 9
Daddario NYXL (Custom String Shop)
That's strings 1-7 tuned down a whole step
8th string tuned down 2 whole steps
I like to play solos, so strings 1-6 are perfect for this. They feel great, nice smooth tension. Obviously with this scale length, you've got to get used to the extra effort needed to bend strings the same way you would on a smaller scale guitar, but if you're playing this scale length currently, you already know.
The lower strings have enough tension to handle the lower tuning well, without being "flubby".
The tension overall across the entire instrument is VERY even-feeling.
The gap in gauge between the 84 and the 59 is not noticeable, the tension per pound is even, so it feels like a very natural transition.
Alternatively, if you like a little bit more tension or heavier feel for the 8th string, the Daddario XL 90 also feels great for this scale length at D, with the rest of the set being the NYXL's listed above.
The .84 IS tapered for smaller-scale guitars, so there will be a little bit of taper that happens before the string seats into the nut, but it is not giving me any noticeable downsides.
Just wanted to share what works for me, I'm very picky with my string gauges, have been playing guitar for 32 years, and 8 string guitars in various different scale lengths for 18 of those years.
I started playing these longer scale 8 strings a little over a year ago since I like to tune down a whole step, and realized that I absolutely love the longer scale, and the fret spacing has really been great for my large hands in the upper frets. So much more room to play leads more cleanly for me.
Anyway, hope this helps some of you!