I'm interested in finding the distance it takes a single omnidirectional pulse of light of a given wavelength under ideal conditions to spread out from the highest possible amplitude to the lowest possibly measurable amplitude.
I intend to after after getting my answer calculate how many digits of Pi are required to use this as the diameter of a perfect circle down to Planck length and pinpoint any point in space and time or spacetime within down to the levels of Planck time and length. However first things first, I don't know how to calculate the distance required under ideal conditions and so require assistance.
Essentially my question is: How do I calculate the distance required for an omnidirectional wave of light of a given wavelength to spread out from the highest possible amplitude to the lowest possible measurable amplitude under ideal spacetime conditions. By 'ideal' I mean flat infinite space free of expansion. The reason for assuming the described ideal conditions is to simplify calculations while also allowing this to be readily adapted to different conditions.