How-to-round-down-the-quantity

What do you mean it has to be rounded down? What position sizing, market, broker provider are you using?

Just for completeness, if you want ultimate control of sizing transactions in your code, the technique is demonstrated here -

CODE:
         int roundIncrement = 10;      // nearest 10 shares
         double size = 253;            // a size to test with

         double roundedSize = Math.Floor(size / roundIncrement) * roundIncrement;
         WriteToDebugLog(size + " rounded by " + roundIncrement + " is " + roundedSize);