I am starting to do some manual back-linking for a medium competition word with high traffic.  I plan to start back linking with like niche blogs with non spam comments, like niche directories, like niche forums, and guest blogging. My domain is over 3 years old and ranks for multiple keywords, but I'm going to concentrate specifically on this keyword, but need to know how many links to back-link a day.  Again none of these will be spam, but will be of real quality. I was thinking 10-20 a day, but unsure.