With variable spread instruments, how do I decide how to set by SL/TP when rates are variable?

We cannot advise you on how to trade and of course, cannot commit to a certain methodology that will promise the Stop Loss will never be hit.
However, we can share a known thumb rule from trading practices. Keep in mind it comes without guarantees:  We suggest setting the Stop Loss far from the market by at least 4 times larger than the typical spread. So, if the spread is 10 pips place it at least 40 pips away from the current rate.