Determining spec depends greatly on the usage case for a product. Imagine you are selling packages of latex gloves. These packages are usually sold by weight to determine how many are in the package without having to count the gloves individually. However, due to this, sometimes there are ±0-3 gloves of the advertised amount. This does not matter heavily in the scheme of things for the purchaser. However, some items like surgical devices have tolerances of ±0.010 inch. One of these devices out of spec may not work properly and may cause harm to a patient. There are two main things I consider the most important when deciding a spec: how that measurement affects product usage, and precision of the manufacturing methods. You cannot make your specs more precise than your machines can make, you must keep in mind manufacturing limitations when designing your product. You must also conduct testing to see how the specs affect your part.