Explain that the way to do estimates is to have a process that starts by breaking down the work into the components of the design, estimating each component, an

Explain that the way to do estimates is to have a process that starts by breaking down the work into the components of the design, estimating each component, and adding it up, with small padding to each granular component. The more granular the breakdown, the more accurate the estimate. For a single top-line complete project guess (which is what you are being asked) you should use a large pad, as comments below say, double and shift. If you can afford the time to do the breakdown, you will get a closer and more realistic number, and have some of the project planning work done to boot.