Calculations and re-basing is done on actual current market value usually. Thats also a good point right there, look to take profits and rebalance your portfolio weekly/monthly/quarterly(whichever you prefer). Perhaps even invest the excess in new coins/risky but high reward ICOs along the way. Another good point is having an amount available in BTC so you can take advantage of market dips and get positions in coins you trust undervalued, which you cant do if all your money is into alts already.