...?

Bitcoin difficulty adjusts every 2016 blocks. A block is mined about every 10 minutes. This means difficulty adjusts about every 2 weeks. There are about 4 weeks in a month.
http://bitcoindifficulty.com/
Bitcoin difficulty is adjusting about +1% per adjustment. Compounded monthly, this means about +2.01% increase per month.

Gaining 20% mining power per month, we are essentially realizing a real-value gain of about 18% per month as long as the chips are delayed.

I think 20% will cut it just fine.



Now on the other hand, if we can get a better deal I'm all for it