1. creat a function to defind how much you can get from 1 pip , due to your entry lotsize
2. creat another function to defind how many pips you gain or loss from your equity
3. and that by your pips-gain-from-equity function you will know how many pips far from current price to mark as a new sl

but this will more difficult if you trade more than 1 order at a time

hope this idea helps you