Anybody here good at math?

the top portion of the image shows how to find the tilex and tiley of a point given the tilesize. tile = int( pt / tilesize ) + 1

I want to perform this same type of operation on an isometric grid given a constant 2:1 tile ratio, and origin is the top corner of the isometric grid.

I'm assuming I should stretch the isometric grid vertically x2 then rotate it counterclockwise by 45 degrees, then finally perform my original equation - but I am unsure how to do this. Can anyone help?