User blog comment:B214/Talk About What You Want! 2/@comment-24098614-20140831091508/@comment-1359322-20140831094111

What the hell are you talking about, more like why the hell are you still asking this. Evil Pieces are completely unrelated to level of powers or limit of powers thing. If it was do you even think Rias could reincarnate Issei at the start of the series. Even if Issei is trash, Ddraig is a Heavenly Dragon, just Ddraig alone would make sure Issei can't be reincarnated by Rias.

"“When reincarnating a human to a Devil we use the [Evil Pieces], but depending on the ability of the person being reincarnated, more [Evil Pieces] will be consumed.”

……So it’s not a confession……

Huh? Piece consumption?

“There’s a saying like this in the world of chess. Queens have a value of 9 Pawns. Rooks have a value of 5 Pawns. Knights and Bishops have a value of 3 Pawns each. Like this, there are standard values that also apply to the Evil Pieces. A similar phenomenon happens to those reincarnated. If there are those who can only be reincarnated with 2 Knight pieces, then there are also those who need 2 Rook pieces to be reincarnated. There’s also the compatibility with the pieces as well. You can’t use 2 separate Pieces to give them different roles, so it’s important to think how to use the pieces. Once you use it, Devils won’t be given a new [Evil Pieces].”"

- Volume 1, New Life

Then there's also the growth of the King.

So Evil Pieces depends the consumption rate.