Excel – Leave Cell Blank Until a Value Is Added

You should be able to accomplish this with the IF function.

The format is

=IF(CONDITION, ValueIfTrue, ValueIfFalse)

So in your case, it may work to do:

=IF(ISBLANK($E9),(I8+E9-G9-H9),"")

ISBLANK() in excel is a built in function to check for what it states. $ is included so if you move over to another column, it still reflects column E. However, don't do that on the number, as that would always reference that number only, rather than following down to E10, for example, if you copy the formula down the row.

To highlight the cells in column G that is empty, you can select column G and click 'Conditional Formatting' (assuming G1 is the active cell while the whole column G is selected) and use the formula =ISBLANK(G1) in the formula box you have mentioned.

Edit:

To highlight the cells in column G that is empty, and only if all of cells A to F in the same row are not empty, you amend the formula as =AND(ISBLANK(G1), NOT(ISBLANK($A1)), NOT(ISBLANK($B1)), NOT(ISBLANK($C1)), NOT(ISBLANK($D1)), NOT(ISBLANK($E1)), NOT(ISBLANK($F1))).

Scott's answer explains well how to highlight the cells if G is empty, and any of cells A to F in the same row is not empty.