In most cases, you want some lower to mana-value cards (to ensure you have things to play early) and some higher mana-value cards so you'll have more powerful stuff as the game goes on
A problem with this test is that there were actually real magic cards printed on blackcore paper as well, they are rare but they exist.
Your opponents often wont pay for this, but if they do theyre forced to stay behind the curve on their turns
Play fetch lands, cheap spells, and a little self-mill like Thought Scour, and youll have no trouble casting Treasure Cruise and Dig Through Time for 1 or 2 mana
The combination of Zaffai and the Tempests plus Niv-Mizzet Parun creates a self-sustaining loop of free spells, card draws, and chip damage that accumulates into lethal
You choose the value of X as that ability resolves