Other things that make it impractical are,

1. The material that the instrument is made of. Most instruments are not made of the same material as music wire. So that leads us to item 2.
2. Temperature. The thermal expansion properties of the instrument are different to those of music wire.
3. Manufacturing tolerance on the diameter of the string. It will be pretty good but then again the diameter is small so the percentage may be larger on thinner strings.
4. Manufacturing variables in the constituents of the steel. These determine the modulus of elasticity of the steel. Music wire used to be high carbon steel but I suspect that modern strings are made from various alloys which could be different for each manufacturer.
5. Is the string plain or wound.
6. if it is wound, what is the weight, size and material of the winding and what are their respective tolerances.
7. How are you applying the tension? If through a regular tuning machine then you would get variables such as gearing and friction. These again would make a determination of the string tension difficult.
8. How accurate do you want your tuning to be?

Take all these things into account and the best of flippin'