my advice on here was for a square metre price, as he is new at the game.
I personally charge £65 per lounge whatever size, (however I do not market the mansion type houses) so it works out fairly well.
~The price is then discounted slightly when more rooms are added to the work.
I never go and quote, If I get it wrong then I swallow the difference... If the customer has lied about anything then I give them the choice before I start to either cough up a bit more, or I walk.
They usually say "can't blame me for trying" and then make a brew and cough the extra dough.
But as has been said I have been doing this for 20 + years