A perfect estimate would have to involve a time machine where you could look in the future to see how much time it took to do the work and what is the cost of the material at that time. So a perfect estimate is unlikely but you can strive to be much closer to a perfect estimate by using the EZestimater Home Depot material update feature. The material database will update twice a week to confirm you are bidding material at the most recent price from Home Depot . Lowes and Ferguson will be added in the future. The perfect estimate will include every piece of material to do the job is accounted for and you can do this with the home depot material updater ie: Water heater installation you need a water heater cost, earthquake straps cost, water heater type D vent , vent cap, copper flex supply lines, gas valve , gas pipe, pressure temp relief valve, copper down pipe , drip pan, shelter , teflon tape. This should include everything . If you don’t include all material in your estimate you will end up donating it. Your labor cost wont accelerate like material so this may not affect your project . But you may want to include an “Escalation of Material” clause in your contract in the chance that your project may take too long to get started and material cost may go up. In my experience as a general contractor I always include some padding in the estimate to cover unforeseen circumstances especially on large projects.