I find 120” a nice cruise gear. So I like that as my second highest gear with one higher 131” for slight downhill/tailwind.
With a 26” rear (normal on your bike) that’s about 53/12 or 58/13 or 53/12 on 406 with dual drive. With the Megarange cassette 58/13 is the second highest but the 58/11 is around 140” You could use the second chainring such as a 53 (a 48 which is commonly paired to the 58 gives you 48/11 which is awfully close to the 58/13) and a use a 30T granny. That’s a difficult upshift, but you won’t need be making it often.
Or, the common 52/42/30 triple works reasonably well with the Megarange, you could optimize later and focus on your build now.
What might work better is customizing cassettes:
If 7 sprocket cassettes, another alternative might be to customize the cassette: take 2-3 cassettes ab & Megarange, use the 11-14, remove 15, 17 of an ab combined with the with the 26 and 34 of a Megarange. 53/39/30 on 26”,
My dual drive 3X7 on 406 with 53T chainring is the equivalent of a 53/39/29 triple on a 26” in overdrive/direct drive/low respectively.
The 11-14 sprockets give me approximately 10 gear-inch intervals in overdrive on the 4 small sprockets (8” and 6” intervals for the DD and low drive positions) with the 14T sprocket sitting slightly closer to the 11T of the lower hub position than the 12T of the same hub position, thereby giving me successive quadruple cassette bank groupings continuously covering the ratio spectrum 55-130 with no gaps and no overlap amongst 11-14T sprockets.
Some of the better permutations I’ve looked at assembling include:
11/12/13/14/20/28/34 minimal ratio repetition with broadest range- made from 3 cassettes
11/12/13/14/24/28/32 ab(11-14) +F(24-32)minimal ratio repetition made from only 2 cassettes
11/12/13/14/17/26/34 ab+Megarange(26,34) w 17T ab separated from matched sprockets
You don’t need a bunch of lows, just an available bailout low, and you need just a few middles to build up speed so big gaps in middle and bottom are fine.
Better to pick close ratios at top because:1) wind resistance means big differences in power translate to small speed changes thereby making fine adjustment more critical at high speed; 2) you’ll spend the greatest time at those speeds; 3) proximate midrange sprocket sizes give duplicate ratios with the other chainrings.
There is a mild benefit to keep adjacent sprockets together from the same cassette for smoother shifting to gain the Hyperglide cassette matching benefit.
You could do something similar with 8 speed cassettes, but since I have a 7 I’ve optimized a lot of nice 7 sprocket permutations.