Variable in xml

I have two variables created in xml. How do I use them to get the third variable that I will use in the future?
In my version, how do I write the third line with the formula correctly?

  	 <value name="F1" offset="4" length="1" startbit="0" bitcount="4" ></value>
	 <value name="F2" offset="5" length="1" ></value>
	 <value name="SpentFuel" units="L" conversion="F1*256+F2" ></value>

In this case!