Another note on precision.
You will be estimating your volumes at different temperatures, and the volume of a given mass of wort at one temperature will be different to the volume of the same mass of wort at another temperature.
You would lose approximately 4% of volume cooling the wort from boiling point down to 60F.
Also, at the start of the boil, you will have some proteins dissolved in the wort and will therefore contribute to the pre-boil gravity reading. During the boil, these proteins will be precipitated as break material, and once precipitated, will no longer contribute to the gravity, even if the sample was taken from the kettle. Of course, if you take the final sample from the fermenter, you will have to account for the cold break, hop and trub absorption, dead space in the kettle, and wort trapped in equipment.
Taking these factors into consideration with my equipment, I lose about 4 - 5 gravity points between taking my pre-boil sample and measuring the gravity in the fermenter.