"Conduit of Ruin" and "Endless One"
Yes, your interpretation is correct. From the card rulings:

Conduit of Ruin’s last ability doesn’t change the mana cost or
converted mana cost of any spell. It changes only the total cost you
actually pay.
If you control more than one
Conduit of Ruin, the last ability of each of them applies only to the
first creature spell you cast each turn, not to different spells; if you control two Conduits of Ruin, the
first creature spell you cast each turn will cost {4} less.
If the first creature spell you cast in a turn has {X} in
its mana cost, you choose the value of X before calculating the
spell’s total cost. For example, if the first creature spell you cast
in a turn has a mana cost of {X}{G}, you could choose 2 as the value
of X and pay {G} to cast the spell.

So, the value that you pick for X is fixed, the only thing that changes is what you actually pay.