How can I avoid an error with a simple mathematical problem?

I am beating myself up trying to work out why the simple mathematical expression
9200071+1000000000000000000 is not adding up correctly in GuiXT.

GuiXT uses 64 bit floating point numbers for calculations. This guarantees precision to 15 places, which is not enough in this case, so I suggest you use the following:

Set V[x] "9200071" // example

// add leading zeros, length 18
Set V[x](1-18) "000000000000000000&V[x]" -alignright