For a more detailed analysis on land percentages I'd refer to Frank Karsten (scroll down to the conclusion if you want just the numbers):

https://www.channelfireball.com/articles/how-many-lands-do-y...

With 35% lands (21/60) you are likely to end up mana-screwed (not enough lands) unless you build your deck to match the low land count. For a generic rule of thumb I think it is better to recommend a bit more lands than that.