Which currency (crypto & fiat) to use to measure profits?

[Ealex]

Lets say you have added enough USD to trade with so you pick BTC to trade altcoins with.
You buy Altcoin X for let's say order total 0.5 BTC
BTC goes up in value but Altcoin X in USD has jumped big time in value
In BTC terms you have lost as BTC's value increase is more than Altcoin X
You sell Altcoin X but you get less BTC, in USD terms you happy, profit both up
BTC drops in value to the USD, you got less than you started with now what?

Do you ignore the USD value and measure only in BTC to get your base currency/crypto currency to use for measuring profits trading Altcoins?

I just cannot get my head around it. Any advice and help/direction will be appreciated.

[secondgarlic]

To get a sense of where I am, I always pay attention to the USD price, otherwise you will always lose where you are. The bitcoin price moves all the time, altcoins fluctuate like crazy anyway, so I always think of it like, how much btc in USD did I sell to invest in altcoins, and how much USD would I make if I cashed out all my altcoins right now. I couldn't find any other sensible way of keeping track of where I am.