Adding 1 cell to another cell and keeping the value and have other cells subt from cell.

I have a spread sheet that tacks printer toner changes. I am trying to incorporate into this inventory tracking. A2 through D92 is printer information and toner change counts along with service call counts. P2 through S26 is printer info with on hand quantities received date and received quantity. I would like to add 1 cell directly to another cell. For example. Q2 is an on hand inventory cell with a value of 5 and S2 is a last received quantity cell with a value of 3. I want to add S2 to Q2 and have Q2 show 8 in the cell ( keep in mind these cells will change regularly due to inventory use and receiving.) Next time I input a new quantity into S2 I want it to add to what is currently in Q2. Then on the other end of it I would like to set it up so that if certain cells are changed they will subtract from this cell or other cells. Example is C2 tracks toner changes for a certain printer, When i adjust this quantity i would like it to subtract from the corresponding on hand inventory cell for the toner used. I have attached the file I am working with. Forgive me, the spreadsheet is a mess but it serves its purpose well! :-)