I suggest the following approach using gear inches:
1. Determien the highest gear inches you are comfortable with, typically 100 to 104. This defines the small cog if your large chainring is already fixed.
2. Detemrien the lowest gear inches you need, typical 27 for sport touring on steep hills and lower if doing loaded touring. This defines the largest cog if your smallest chain ring is already fixed.
3. Pick the casette with the above high and low cogs and ensure the intermediate cogs are properly spaced.
A triple may provide the following:
The needed granny chain ring while giving closer spacing on the larger 2 rings.
Allow for a smaller large cog.
John Hawrylak
Woodstown NJ