Once a month I enter a value (currency) into a row of cells,
What is the formula for returning the sum of all the cells in a row which are populated, ignoring cells which are currently empty but will be filled at a later date?
Just use SUM() over the range (e.g. =SUM(A2:A999) or whatever range), it ignores empty cells (and even if it didn’t, adding 0 for empty cells wouldn’t change the result anyway).
See SUM Function.