And queue the comments about IBUs not mattering, it is all about perception of hops. That is precisely where the ratio comes into play!
Say I'm making a Cascadian IPA (It isn't a Black IPA, change my mind!) and I go heavy on the Chocolate Malt. It doesn't matter that I have 65 IBUs if my ratio is too low. If I'm not mistaken, this style needs a bitterness ratio of 0.78 to 0.98 or so. Because I went heavy on the dark malts, my BR might be as low as 0.500, well below what I actually want for the style.
Above I think I said the Blond was 0.030 and 0.040, I meant 0.200 and 0.400. I have two sick kids and I'm balancing replying and caretaking. It is distracting.
Anyway, is it a sin to link to BeerSmith? Here is a great blog post:
https://beersmith.com/blog/2009/09/26/balancing-your-beer-with-the-bitterness-ratio/