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.