There isnt a perfect ratio, but for larger sites it is very important to get a large number of backlinks to internal pages. It helps with the overall "Domain Authority". If your site is structured well, then the "seo sculpting" internally will help boost the homepage. You should make sure to utilize internal links back to the homepage or important high-level pages.
For instance, lets say you have 10 backlinks. If they all go to your homepage, your homepage could have a nice SEO value with a pagerank of 5. But your interior pages dont have much pagerank or seo value, and therefor the rest of your site doesnt get ranked well.
In contrast, lets say 8 out of the 10 links go to internal pages, and those internal pages all have a small pagerank, and those pages all link back to your homepage with important keywords, it will pass a bunch of that seo value up to your homepage at the same time of increasing the value of your internal pages. Its a better way to get your whole site ranking, rather than just homepage.