Time Required to Write an Article

I recently completed writing two articles from scratch for two different pharmaceutical companies. One was a strightforward paper reporting the results of a preliminary trial of a new drug while the other was a review of the client’s drug compared to similar drugs. I wrote the former based on a collection of tables and graphs that the company provided summarizing the results in a rough form, along with the protocol, while the latter was based on reading a whole bunch of articles. Both came in at about 6000 words total including references and abstract. What was interesting was that both took me about the same amount of time; around 85 hours. Before I start a project clients always ask for an estimate of how many hours will be required and I am interested in refining my estimation techniques. My estimate for the review was more or less accurate but I underestimated the research paper. I would be interested in hearing from other writers about the techniques you use for estimating the hours required for writing different types of papers from scratch. Thanks