I usually turn the buy/sell signals into 1s or 0s then use the ValueWhen function
to calculate the profit/loss, then go from there.

Try something like:
When you have hit your buy signal and you want the last profit/loss :

ValueWhen(1,sell signal true,C)-ValueWhen(2,buy signal true,C) will give you the
last signal.

Play around with the "1","2" in the ValueWhen to grab the correct signal.
Refer to manual for more info.
I code like:

b1:=buy signal;
s1:=sell signal;
pl1:= ValueWhen(1,s1=1,C)-ValueWhen(2,b1=1,C);
d1:=If((pl1>0),do this, otherwise do this);
d1

You may or may not need BarsSince() to confirm the correct buyy/sell signals for calc.