the more detailed you are, the better.
when i first started estimating, i was way off base. looking at what my boss did and doing time studies to see exactly how long it took to do specific jobs allowed me to get better +/- 5%.
i have moved on to bigger & better things and still do estinating. create a worksheet with common practice items, then assign the associated costs and labors for that item. next create wiring lengths and costs.
compare that to jobs already done. find out where you missed is and why. then fine tune it and compare to other jobs. after several trys you will get pretty close. repeat the process for all the jobs and see hoe it turns out.
