MT5 Arbitrage EA Guide: Pairs Signals and Execution Limits

#property strict
// Signal-only Expert Advisor: no orders, positions or profit calculation.
input string Symbol2="EURUSD";
input int SpreadLookback=100;
input double EntryZScore=2.0;
input double ExitZScore=0.5;
datetime lastBar=0;
int OnInit()
{
   if(Symbol2==_Symbol || SpreadLookback<2 || SpreadLookback>10000
      || EntryZScore<=0 || ExitZScore<0 || ExitZScore>=EntryZScore)
      return INIT_PARAMETERS_INCORRECT;
   if(!SymbolSelect(Symbol2,true)) return INIT_FAILED;
   return INIT_SUCCEEDED;
}
void OnTick()
{
   datetime current=iTime(_Symbol,PERIOD_H1,0);
   if(current<=0 || current==lastBar) return;
   MqlRates a[],b[];
   ArraySetAsSeries(a,true); ArraySetAsSeries(b,true);
   int count=SpreadLookback+1;
   // shift=1 excludes the forming bar. Index zero is the newest completed bar.
   if(CopyRates(_Symbol,PERIOD_H1,1,count,a)!=count
      || CopyRates(Symbol2,PERIOD_H1,1,count,b)!=count) return;
   if(iTime(Symbol2,PERIOD_H1,0)!=current) return;
   double spreads[]; ArrayResize(spreads,count);
   for(int i=0;i<count;i++)
   {
      // Reject mismatched timestamps; do not pair unrelated bars by index.
      if(a[i].time!=b[i].time || a[i].close<=0 || b[i].close<=0) return;
      spreads[i]=MathLog(a[i].close)-MathLog(b[i].close);
   }
   double mean=0;
   for(int i=1;i<count;i++) mean+=spreads[i];
   mean/=SpreadLookback; // Historical window excludes the evaluated observation.
   double sum=0;
   for(int i=1;i<count;i++) sum+=MathPow(spreads[i]-mean,2);
   double sd=MathSqrt(sum/(SpreadLookback-1));
   if(sd<=1e-10) { lastBar=current; Print("No usable spread variation"); return; }
   double z=(spreads[0]-mean)/sd;
   lastBar=current;
   string signal="inside thresholds";
   if(z>EntryZScore) signal="candidate: short first / long second";
   else if(z<-EntryZScore) signal="candidate: long first / short second";
   else if(MathAbs(z)<ExitZScore) signal="near historical mean (not a position exit)";
   Print(TimeToString(a[0].time)," ",_Symbol," / ",Symbol2,
         " z=",DoubleToString(z,4)," ",signal);
}