by EmCee » 21 Jan 2011 6:09
Assuming that you're asking only about cylinders used in uPVC doors, in an ideal world you carry all the sizes, in Euro and oval profiles, double and single, brass and satin finishes. Yes, that's a lot. With experience of your local area you might discover that there are some common sizes, but that'll still leave you with a lot of less common sizes that are still used in the area.
Forgetting the 'ideal', all you can do is spend as much as you can and increase stocks as you build up your business. Having all the sizes means you can fit a new cylinder of correct size and finish in one visit. If you don't have the right size/finish you will have to return when you've ordered one - that's something you might have to live with to start with, but it obviously affects the profitability of the job based on your hourly rate.
Cheers...