Nested For loop Grouping

Hi Everyone,

I'm making a procedural City generator using nested for loops;

public Regions[] regions;

 for (int x = 0; x < gridX; x++)
 {
 for (int z = 0; z < gridZ; z++)
 {
 if (x < gridX/region && z < gridX/ region)  <--- what i need to replace
 }
 }

I have done quite a bit of work on it now but what i want to add next is regions, and i cant figure out the math in my head.

In the example image I could just get the x and z's below and above half but I would like to be able to add as many regions as I want and for it to calculate groups similar to the image above.

maybe using a percentage or something? i dunno my mind has gone black from looking at it so much...

Thanks in advance!