Since you always want to subtract the value of the text-entry box from a consistent value in Variable 1, don't do the calculation within Variable 1. Instead, do the math in another variable. For example, create a variable called Calculation. Then use this series of triggers to set its value:
- Adjust Calculation to equal Variable 1.
- Subtract the TextEntry value from Calculation.
Those triggers will always subtract the current value of the text-entry box from Variable 1. And that means you don't have to add back a value that was already subtracted.