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I indeed use the trade amount in the fee-overcome calculation. But i do not use in the way you think. You are totally right i should use 1 BTC because the fee-overcome looks the the current prices (and those are based on 1BTC) but this is only true for 1 BTC and up. Below 1 BTC the assumption will go wrong, because at that moment the trade-amount becomes leading in the calculation. To explain it more easy, the needed market change is smaller because of the smaller trade-amount.
That's the part I disagree with; the needed market change is
independent of the trade amount. Whether I trade with $10, $100, $1,000,000, the needed market change is the same. This is because the fee is charged as a %, it may be easier to follow the maths I posted above though. In all cases I need the price to go up by a factor of 1/(0.998^2) if buying then selling, or down by that factor if going the other way around. Hope that makes sense.