What is a good basic/non-basic lands ratio for Back to Basics in Commander?
Really it depends a lot on the meta you expect to be playing in more than the number of colors in your deck. There are cards that create a problem for you playing non-basic lands, like:


Ruination
From the Ashes
Back to Basics
Blood Moon
Detritivore
Dwarven Blastminer
Dwarven Miner
Helldozer
Trench Wurm
Shivan Harvest
Trailblazer&#39;s Boots


In that case your non-basic lands are going to be a problem, and this doesn't include non-repeatable effects that work on individual lands like Fulminator Mage and Wasteland. You could also have an issue when given the chance to look for basic lands if you have none, through effects like Path to Exile, Veteran Explorer or Collective Voyage.

However this doesn't mean you should go with only basic lands either, even if your deck is monocolored. There are also cards that can cause you issues with using basic lands, like:


Reality Twist - except for blue, Boros and Golgari
Infernal Darkness - except for black
Contamination - except for black
Naked Singularity
Sundering Titan


And for specific colors:


White - Stench of Evil, Flashfires
Blue - Boil, Boiling Seas, Tsunami, Omen of Fire, Choke, Curse of Marit Lage
Black - Karma
Red - Glaciers, Volcanic Eruption
Green - Acid Rain
One basic land name - Wake of Destruction


All of which create problems for basic typed lands, and these are just the biggest ones, there are others that just do damage for tapping land with a specific basic typed, like Scald or ones that gain the controller life when their opponents tap that land like Lifetap. Players using the upkeep spells here will also likely play Eon Hub to bypass that cost. There is no perfect answer here, there are cards that are a problem for both sides of the basic/nonbasic equation. Based on this going all one way or the other leaves you open to more problems