Yes you only have to calculate it once and then apply your base figure to each job.This way you will find the minimum you could reasonably afford to do the job at.
Its more crucial for the larger jobs than most domestics but should be seen as good practice for everyone.
Its very similar if you do a business plan, it shows you where you are now, where you want to be, and how long it will take you to get there.
As your costs change you should change your rates.
To remain competative you may have to cover some expenses out of your profit.
If you have a clear understanding of how much it costs for one hour of work, then you will appreciate how much you have to earn to make a living.