Open position using whole free margin

Hello guys,

I've tried this way:

double acFree = AccountFreeMargin();
 
int acLev = AccountLeverage();
   
double maxVol = ((acFree*acLev)/Ask)/10000;
                 
int ticket=OrderSend(Symbol(),OP_BUY,maxVol,Ask,3,0,0,"expert comment",255,0,CLR_NONE);

but never whole margin is used - always 3-8% of margin remain unused.

How to make it?