How do you gauge weather a stove top custard is thick enough for a no-bake custard pie such that when it gets sliced( after cooling) it would not be too runny?

i cooked my custard till it coated the back of a spoon and left a trail when stirred but it was still too runny. it definitely was thick, but not slice-able, i would say more suitable for individual tarts

does the custard need to pull away from the sides like a choux pastry? i was worried it would become too firm.
any help greatly appreciated!

btw, what is the difference between a custard and cream pie?