[Gen 3 RS] Find eggs initial seed

[glu8716]

When abusing the RNG of Wild and Static Pokémon it's pretty easy (with the "Seed to Time" tool) to find what initial seed and how many advances you need to hit the target seed. This is really useful as with paintings or ACE we can then manipulate the initial seed.

I think it would be pretty useful to have a tool that can do the same for eggs. At the moment of writing this post in PokeFinder when searching for eggs in Ruby and Sapphire you must start by setting an initial seed. However I would like to do the opposite: first searching for the target, and then get the needed initial seed for it.
I know we can just type in random seeds and something will pop up. But if a particular target is to be found on a high value of advances (such as 30000+) I've noticed that PokeFinder is pretty slow with the calculations and this is not really convenient.