If you're inserting the links to improve user experience why does it matter? Just insert the links where it makes sense.

If I understand your question correctly you're asking what is the best way to manipulate Google rankings with your network of blogs? Add links to old articles or write new articles?

My answer would be - on the new articles.

And bring down your network of blogs. It will come back to bite you one day.

Not really. We go by "feel" most of the time too. If you know they're not willing to pay as much as you think it's actually going to cost 'hours' wise, sometimes you just have to charge what you think they're willing to pay. Obviously, if you do this for too many clients you'll loose out, but if you're really slow it can make sense.  Moral of the story, try to go for bigger clients :). The smaller ones usually end up burning you if you're on a Cost Per Hour model....in my humble opinion at least.

From my experience it's much easier to go back to the well with larger clients. Smaller clients don't tend to appreciate the "margin of error" bit so I'll try to over-estimate the quote when dealing with them. Smaller clients tend to ask 2x as many questions as well.

10-20% sounds about right for project management, but I won't put that in the proposal. I'll mark project management and reporting as "free" and just include them in the package (with the markup in other areas). I don't put the hours estimated for each task either... though i know some agencies do.

That doesn't 100% answer your question, but I hope it can help.