Steve, totally agree with you, baffles me how some folks walk in to a job and pluck a figure out of the air! Doesn't look very good to the customer either, the measuring up is pantomime if nothing else. I even get them to put their foot on it when they offer (even though it's unnecessary and a pain in the @rse!) and I'll announce each measurement as I take it!
I think more times than not when trying to work out the area of a bigger job, people will under-estimate rather than over-estimate, so it's not like it balances out in the long run.
John you need to persevere with those quotes mate. It's a hell of a lot easier to sell to people face to face, by being there and inspecting, measuring up... a phone conversation is just a price, no good.
You have to work on your sales procedure when in home, and also remember you can suggest protector or additional rooms in a subtle way which can turn a £100 job into a £150 job. That's your week's diesel paid for, just for getting off your backside and being bothered to put the effort in

You don't have to be a slick salesman type, in fact I can't stand that sort of rubbish, I'm just really friendly helpful and honest, and subtly put the idea of extras in their minds rather than a full-frontal sledgehammer approach to upping the job ticket. [If you make people like you first and foremost, selling to them is easy. Work on that

But here's the thing.... you have to get good at deciding which phone enquiries are worth the home quote and which are going to be time-wasters and just get a price then "I'll get back to you".
If you filter your enquiries this way you'll find you end up converting 90% of home quote visits, because you've already filtered out the ones you don't want.
Sometimes I am surprised and a very quick "price" conversation results in a booking... also the other way round, a job I felt sure to get from a home quote didn't materialise for whatever reason.
That's business unfortunately
