am I understanding X correctly ?

[codifies]

take for example Pest Infestation

There is no definition of how X should be calculated, so I can choose, but say I want X to be 4 then as the cost is (X) (X) (G) then the mana cost for the spell would be 8x colourless and 1x green, 8x 1,1 B/G token creatures  would be created and 4 artefacts and/or enchantments would be destroyed.

[Morganator 2.0]

Yes, all of that is correct.

[codifies]

Thanks!