If there are multiple systems and the SFD Graphic is complex, this is likely to be a 1% rounding error, which is unavoidable with so many calculations. If it is larger than this, then it is more likely to be an error in the input data and users are advised to check their working. Where a small rounding error is encountered which makes understanding difficult and/or delays agreement amongst stakeholders, then users are encouraged to make minor adjustments in the estimated percentages to eliminate this.  If this proves difficult you can contact the SFD Helpdesk who will endeavour to identify the problem and provide assistance with making the most appropriate adjustments.