I only really use milestones for early wonders and religions.
For standard Emperor:
-Stonehenge/Pyramids is safe until t25, a bit risky until t30 and very risky after that.
-Petra/Artemis/Mausoleum are safe until t40, a bit risky until t50 and very risky after that.
-For religion, <t90 is likely to make you the first founder, t90-110 is foundable but you might miss out on tenets you want, t110+ is possible but risky, with t120+ I just don't bother. I adjust my expectations based on civs (especially if India/Spain/Celts are in, you can tell without meeting them) and look at your neighbours' pantheons.