Estimation techniques used in embedded porting projects

Hi all,
This is slightly an offtopic question though related to embedded
projects:

We have to work on porting projects.We till now are not aware of a
good effort estimation to arrive at for the porting project.I would
like to know what techniques industry uses or you experts have
followed while doing effort estimation for embedded software porting
sort of project with RTOS and couple of tasks.The biggest difficulty
we face is in conveying to the client the needed time based on perfect
estimates.

Looking farward for your replys,
Regards,
s.subbarayan

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