Totally agree with Andys first reply.
This is how I deal with it.
If it it domestic, then it is a sq mt rate, as we all know that domestics can take longer then a commerical job for it's size. Domestics are hard to charge by the day, bad drainage, low water presssure etc, so time lost.
The only time I'll give a day rate is it is commerical or industrial, (market dictates this) £450.00 per day and this at times is a low price.
For that price you'll get me, machine, any chemicals needed, and a set amount of sq mts to be cleaned. Now saying that if the said area (square meters) is, say a day & a half of my time you will still be charged 2 days, end of it. I am charging what it is worth to me to clean said area, and these will be mostly one off cleans.
I would also say stop feeling guitly for what we/you charge you are offering a professional service, with professional kit not just an add-on with some B&Q or eBay cheapie bought kit.
The sooner people realise that there are professional companies out there the better.