Hi Mike,
I know what you mean, the trouble in Spain is some of the home-owners are only there for a few months of the year. You don't want to loose the account because its going to be an all dayer (Anyone else reading this from England will be scratching their heads now), but you know that 600 odd windows isn't going to be a 2 hour job, especially after the cleaning woman has scratched every one of them with her spray & old rag.
I usually ask when they were last done, & figure how many hours it will take me. I take the owner on a tour of there house, & show them where they have been scatched & the problems of each cluster of windows. Once this has been achieved, I tell them how many windows they have approxamately & then give my price, and the blow is halved. I then go on to explain that they could use another uninsured window cleaner, etc, etc, but my approach seems to work, & tell them that the job will be cheaper next time on a regular basis.
I know you haven't got a water-fed yet Mike, but this does halve the time on the next clean by quite a margin.