There really isn't a defined answer, but Dave's answer comes closest.
Basically time is money, simple as.
Main thing you need to decide on first is the minimum you need to earn for a days work, and this differs up and down the country.
Some need £200 plus, down to basically anything they can get.
So say the average is £130 a day minimum needed, and your pricing up a full bathroom that's 20 sqm wall tiling, then you'd possibly price up on sqm rate, say £20, so for the whole room you'd be quoting £400 labour for the tiling, and it took 3 days to tile, you're looking at a day rate of £133 which is just over what you really need.
But if you then looked at tiling an complicated 10sqm kitchen with loads of wet cutting round 12 sockets, 2 windows and a extractor hood in tiny 10cm x 10cm tiles and whatever else and it was going to still take 3 days to complete, you wouldn't charge sqm rate because you would only end up earning £66 a day over 10 sqm.
Basically smaller tiles equals fitting many more tiles than fitting larger tiles, equals taking longer to fit the same amount od sqm than iot would with larger tiles.
Different tiles can mean the difference between dry cutting and wet cutting, and wet cutting takes a lot longer than dry cutting.
So in essence, the longer you think it will take to do the same amount of sqm area, the more you need to charge, but theres a trade off too this, being new to the game isn't an option to charge more for the sqm area because your slow and can't tile as much as someone else in the same amount of time because you'll never get a job in the first place thinking like this.
Earlier on when your starting out, you will need to earn less to start with because your not as fast as other established tilers, and your going to need more time to complete a job because your a new guy, but speed comes with experiance, so the first few jobs will take longer than you expect it to, but you'll get quicker through experiance and finish jobs in half the time, doubling your day rate.
I hope this all makes sense, Im quite........ over the limit at the moment
