Peripherals that transmit as much data as they receive have 50% writes; the spreadsheet assumes the remaining 50% of the time is spend on reads. In some applications, peripherals move data in only one direction, or have a known balance of data movement. In these cases, % writes should be changed to 0%, or 100%, or the known ratio as appropriate for the cases when the SoC is reading all the time, writing all the time, or a combination of the two, respectively. Otherwise, 50% is a typical number that should be used.