Mining have nothing to do with the btc/usd price ratio. You should always report mining only to btc.
I understand what do you mean
so we must stick in BTC for mining report, not USDif we do like that, hard to make ROI in 3 months, maybe 4-5 months still possible
That is exactly it. You always calculate in BTC (invest) to BTC (mining profit).
If you're mining (hardware or cloud) hoping USD rise, but with a risk of not making as much BTC as you put in, you're losing money plain and simple.
If you invest in USD, but are also hoping for a BTC price increase, you're better off just buying the Bitcoin outright and sitting on it.
You're always taking a risk, which is what my initial statement was. Regardless of if you invest USD, EUR, YEN or BTC, you hope that the output will be greater than your input. If you invest 4
BTC now, you're hoping that difficulty doesn't rise so dramatically as to make it impossible to recover that 4
BTC. If you invest $2599 now, you're hoping that the amount of BTC you earn from mining ends up greater than the $2599 you put in.