Hi there,
There are so many variables. Besides the fact that it's technical and that the subject is relatively unfamiliar, you should also consider whether there are numbers/statistics to double-check; whether there are graphs or charts of any kind; whether there are notations, endnotes, etc.; and whether the technical jargon is usually full words or acronyms. (I find acronym-filled sentences harder to proof). And that's on top of the biggest variable: whether the person who wrote it has done a terrific job of proofing already. Unfortunately you probably won't know that last one until you've seen the document firsthand.
I've been editing and proofing other people's work for more than 15 years, and I STILL tend to underestimate time requirements. So what I've finally learned to do is tally up a number based on 5 pages/hour (might want to use 3 pages/hour for technical). Then when I write up the quote, I
double that number. That's the only way I can stay in the ballpark. In fact, that doubled number is usually far closer to accurate than my original thought.
The nice thing about doing it that way is, there are no nasty surprises. If it really does take that long, it's OK because it's what the client expected. And if it takes less, the client will think you're a superhero.
If you're very concerned the client will run away when he sees your honest estimate, you might offer a flat-rate deal as a sort of trial. I've done this before. Let's say your client is willing to spend no more than $25. Work out how much time that will buy at your rate, and start proofing. When $25 of time is up, stop. Even if you're in the middle of a sentence, STOP. Then send the client an email telling how much of the document you got finished, and attach a small sample of the proofed section. If it turns out the document was already well-proofed by the client himself, he may decide not to continue. But my experience doing it this way is, once the client sees the difference, he realizes how much he needs the full proof and decides to have me finish the job after all.
Whatever you decide, DO NOT be tempted to underestimate. Remember, you're worth every dime.