Never given an unlimited 'day rate' where I don't know how long it'll take. Would look very bad imo saying "I'm *** a day but I'm not sure how long it'll take". Usually when pricing a job I will measure up and work out how long it'll take vs the meters, and give a finished price. Usually the meterage and dayrate works out about the same, apart from on big floors where I can always pull in a better price compared to a bathroom. If there are unknown factors, I will give a best case and worse case scenario price eg, standard tile removal only in a bathroom and complete reboard. Customers are always fine with this as they know their parameters, and if I make a balls of pricing it then I will take the hit myself.
Still get caught out the odd time but in 95% of jobs I will be able to tell how long it'll take me and if I finish before that then it's a bonus. To be honest I don't like giving my 'daily rate' to customers as they may think it's too high for a lowly tradesman, a fixed all in price for labour and a separate price for materials seems to work best.
I have a base line and as long as I meet that every week I'm happy. Eg, had 120m in a house last week. Had given the same price per meter for bathrooms and floor. They changed the tile for the bathrooms to a 8x4 ceramic (showers only and 600x300 floor tile) and we were running at a bit of a loss due to the slowness of fixing the wall tiles. Wasn't worried as the 90m very flat floor downstairs would swing it back again. I could have been grumpy and told him we needed more money for the walls but there's no point being greedy, as at the end of the day there was still a good wage in the job overall.