Google Sheets - If statement to populate cell - if-statement

Question

It feels like this should be really easy, but I keep getting errors related to circular logic.
Column C "Total" will always be entered by the user first. If user enters number in Column B "Variable" then Column A "Fixed" will be populated with C - B. If user enters number in Column A "Fixed", then Column B "Variable" will be populated with C - A.
https://docs.google.com/spreadsheets/d/1xBbU6A_MDK6fyLjdFUD7X06b7BQ1VhQ-FWQBET4cLso/edit?usp=sharing

Answer 1

You are trying to add formulas that will always need to rely on eachother to produce an output and as result of that, it will run into a Circualr Dependency error.
Possible solution:
Try using the "Iterative Calculation" option under File –> Spreadsheet Settings –> Calculation. You can see the description for Iterative Calculation here.

Answer 2

Here is one way to avoid circular references: do not hand enter any formulas, but use an onEdit() script to insert formulas programmatically only when necessary.
The following script will enter a formula in column B when column A is edited, and vice versa:
function onEdit(e) {
if (!e) {
throw new Error('Please do not run the script in the script editor window. It runs automatically when you hand edit the spreadsheet.');
}
const watchSheet = /^(Sheet1|Sheet2|Sheet3|Sheet4)$/i;
const watchColumns = [
{
colNumber: 1,
formula: '=C:C - B:B',
},
{
colNumber: 2,
formula: '=C:C - A:A',
},
];
const sheet = e.range.getSheet();
if (!sheet.getName().match(watchSheet)) {
return;
}
const editedColumn = watchColumns.filter(column => column.colNumber === e.range.columnStart)[0];
if (!editedColumn) {
return;
}
const updateColumns = watchColumns.filter(column => column.colNumber !== editedColumn.colNumber);
updateColumns.forEach(column => {
sheet
.getRange(e.range.rowStart, column.colNumber)
.setFormula(column.formula);
});
}