If for example I am trading DASH which is rising in price as BTC falls, is any profit i make on Dash rising balanced out by BTC falling as ultimately it will need to get traded back into BTC?
just make it simple, assess your trading profit one step/phase at a time
you start by converting your fiat capital into bitcoin amount, write down your initial $ amount
if you are trading btc/dash pair then you buy dash when it's low and sell when it's high
so to profit in altcoin trade is to accumulate more btc than you have before trading
then the next step is to profit in fiat term,
to profit in this phase is to sell your btc for more than your initial capital fiat you invested earlier
so your $ amount now should be higher than initial $ amount, that means profit!