Estimating fees

Many prospective clients will not appoint us unless they have a fixed fee. It is claimed that fixed fees are the way to bigger profits. In practice I find it more or less impossible to estimate the fee, unless it for the likes of building s/c, and so do not take on some clients.

Can some of my fellow small practioners comment upon how they take on new ( not recommended) clients when they demand a fixed fee.

Also what rule of thumb do practioners use to set their estimated fees?
Robin Stewart