Return value of 0 if value is less than zero
The scope of each of the two different and unrelated voltadj variables that your code (as below) declares is limited to within its enclosing set of braces.

if (voltage-.47 &lt; 0)
{
    float voltadj = 0;
}
else
{
    float voltadj = (voltage-.47);
}


Instead perhaps say

float voltadj = voltage-.47;
if (voltadj &lt; 0) voltadj = 0;


or perhaps

float voltadj = voltage &lt; .47? 0 : voltage-.47;


or

float voltadj = constrain(voltage-0.47, 0.0, 5.0);


or 

float voltadj = max(0.0, voltage-0.47);


Also see max() and constrain() reference pages at arduino.cc.