Useful Job Time Estimation Calculation

I've figured out a way to realistically estimate just how long it will take to do specific jobs:...first-off, estimate how long it will take to get hold of the things you need (allowing for delivery time if buying online); factor in prep time (stripping paint, grinding or drilling out spot welds and removing rusted metal etc) usually one or two days, then allow one or two days fabrication; cutting and measuring replacement patches and panels and then finally apply an extremely simple mathematical equation (you need only very basic mathematical knowledge to do this)...you either multiply your estimate by 3 or alternatively, multiply it by 4..or possibly 5; very worst case, multiply by more than either 3, 4 or 5.