Threshold is an ability word, so it should be spelled out on the card, not given reminder text.

It was really difficult for me to word the card correctly; I may need some help with this.

"As long as seven or more cards are in your graveyard, if a spell is countered this way, add X mana to your mana pool at the beginning of your next main phase, where X is that spell’s converted mana cost." You're right, that's ugly. What if instead it always gave the mana at your upkeep, but threshold prevented your mana pool from emptying that turn? "When ~ ETBs, counter target spell. At the beginning of your next upkeep, at to your mana pool, where X is that spell's CMC. // Threshold - As long as seven or more cards are in your graveyard, during your next turn, mana doesn't empty from your mana pool as steps and phases end."

Alternatively, you don't have to make a strict reprint of Mana Drain. You could always opt for the mana to be added to the pool right now, and have the player figure out how to best take advantage of it.

Update: I merged both of your suggestions together. Thanks.

I can't tell from this wording whether the mana would empty at the beginning of your next main or the end. Why not let it not empty until the end of your next turn? It won't make the spell much better.

That seems fair. I'll update it.