Hi Jim,
When I started taking on bigger stuff, I was honest with the person who was organising the window cleaning, I told them that I hadn't done much of that size of work and that my price might be a bit up or down from their last cleaner. However, after I'd done the job a couple of times I would re-price the job at the time I could do it in once I'd got used to their premises.
Some were good enough to tell me what they'd been paying the previous firm - if it was lower than my price, I'd ask to raise it - if it was higher than I would have charged, I agreed that that was a fair price!
Do a bit of asking when you meet them - Who was your last firm? (They might not tell you, but if you know who the cheap and dear firms are you'll get an idea of what to ask for) Why don't you still have him? (If he's deserted the job - why? are they bad payers, is there something about the job he's not telling you. If they got rid of him - why? are they exceedingly picky? etc, etc.)
Who Where What When Why and How are your best friends when you're trying to suss out a new job.
Just be straight with prospective customers. If you've done the same style of house/building before and you think you're a bit out on price - correct it for the next one in that style. If I wasn't sure about a new house I told the people that, and that I'd do the job and then give them the proper price. (You always take a bit longer on the first clean anyway!) If they weren't happy they could pay half and we'd part friends. This very seldom happened, but you knew the real price for the next job that size. "Oh yes Madam, we do your neighbour/s already - we'll just charge you the same as they pay."
All the best for your new venture - you've come to the right place!