How do you estimate development time?

I have a question that probably gnaws at most development houses at some
stage - what is the process you use to estimate the number of hours
you're going to need to complete a project? 


Do you use a method that boils down to "we had a similar job to that
last month and that took us 55 hours and this job has that extra tricky
bit so it'll probably take us 58 hours"  or are you more scientific than
that?


I have to change my methodology for billing my major client from a
hours-worked basis to a job-by-job basis which means I'm going to have
to be more accurate in estimating how much a job is going to cost me.


Cheers,
Michael Kear
Windsor, NSW, Australia
AFP Webworks.

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