if you need a rise in btc prices to roi your miner you shoulnd`t buy it. just buy some btc and hold it. buying a miner only makes sence if it wil produce more btc then you can currently buy with that money.
+1
although if you don't plan on buying with fiat but the price drops suddenly like it is now and you decide to invest you could calculate your ROI starting from your initial fiat investment. I know, it's just mental acrobatics but still...I don't like to buy low but I like to invest low.