Returning Blank fields

Hi, I've created a very simple description template of what I'm trying achieve. The number sold looks up from the sales team Total for a quarter & then column F works out the stock sold as a % of the total stock held. What I need to work out is how do I get fields that have no data in them Q3 & Q4 to return a blank field so that the total % F9 isn't calculating the result by dividing the toal by 4 when there is only 2 quarters of data. (I hope that makes sense)