Use the pi rule. Multiply the estimated duration by pi. Works for many other things too, besides software.
- One to work out what you should be building ("I have to tel you what I want? I might as well build it myself")
- One to build it
- One to get in working in the required way (allowing for "That may be what I asked for, but it's not what I want")
And a bit more for contingency...
NB The above approach is based on an assumption that the real difficulties come from working out what to build - technical risks are generally easier to address (prototypes etc.).