I love it when clients give you the tone they want- conversational, highly technical, comical, positive, evil- whatever they want. But when they don't give you a tone, what should you do? I use one of two techniques, depending on the situation. I can either analyze what I know it's going to be used for and set the tone to what those readers will likely want to read, or I use my "magazine tone."
Figuring out the context can be helpful, but sometimes the client had something else in mind that you didn't now about. Normally, I don't discuss much about what a client asked for or told me, but since this guy never paid, I feel pretty free to do so. He contracted with me for a handout for office workers. Now, not being given a tone, but knowing that it was for office workers, I settled on a fairly straightforward style that was something that a high school senior could read easily. He ended up wanting it redone, saying that he wanted it at a 5th grade reading level. He then sent samples he had gotten from another freelancer and directed that it be written along those lines.
The samples were indescribably bad. Really, really, really bad. They were nowhere near 5th grade level- more like 2nd or 3rd grade. Something like that would be insulting to an office worker. When I worked in an office, if I had been handed something like that I would have been pissed. I reworked the thing to make it as simplistic as possible, but it was almost painful to do so. When the client wants something that you know isn't going to go over well, you just have to bite the bullet and do it, even if you know that the tone and complexity should be something else. But, without a guideline or any direction, I had done the best I could. Usually simply looking at the potential audience works very well.
My magazine tone, as I call it, is for when I am given little direction and don't really know what the articles will be used for. That is a tone that is technical enough to give the work credibility, but is conversational enough so that it doesn't feel inaccessible. This tone is what most people end up wanting for their work, and it fits in probably 90 percent of the work I do. I think it's a good, readable tone that works for most web writing.