High frequency trading could cost you too much time. Normally I just do some research on one coin, buy it, if it rise more than 20%, I sell it and change back to BTC.
If you can buy in dip price (or flash crash), ratio 20% is enough for take profit like your said, because I can sure 100% any altcoin price after dumped and change -20% to -50% always have rules comeback to old price or near open price.
It is a good choice to buy altcoins from the dip price and a 20% profit from your capital is not bad and a 100% return for your capital is too high and risky to wait. If you are going to buy coins then just buy those coins who are not on the trend or not hype yet and observe their development on their threat to find news or updates that can cause a hype for the price.