Please help me with a formula, or conditional formatting.

i have the situation: a1 = start date, a2 = due date, and 4 cells with
percentage from done (25%, 50%, 75%, 100%)
i want to do: if the percentage from done is not realised in 25% from start
date then make red that cell. then do the same for 50%, 75%...

if i wasn't clear enought please tell me.

thanks.

adrian.