there's no safe ratio anymore...
I used to be a firm believer in this thing, but a few years ago I saw a website with 100% d0follow links getting heaps of traffic (like, 3+ million visitors a month if memory serves me well), so this ratio is not important anymore if google trusts your site and - probably more importantly - if you are a big brand who gets their traffic outside of google, google will make exceptions for you like they always do with the big guns...
If you do want to be pedantic about it and / or are new to the game aim for 60-80% nofollow links to the 20-40% d0follow ones... that's how I used to do it back in the day (6-7 years ago)...