To colour at interval,to autofill skipped cells

[Freemannew]

1.

On column A any text/value in cell A3 is coloured red.if column C will be up to 30th cell or more, at every 30th cell the value will be coloured red and the value 0.879 in the formula will change. Meaning that it can be less than 30, 30, or more than 30.
How do I write the formula
2. How do I autofill all cells A5, A8, A11, A14, A17 etc