I don't use running totals, DBlank is the expert there. Quite often, CR won't allow certain function to work if they contain functions like Sum as the result is not predictable at the time the report designed...It takes a while to get, but if you can't 'look' at the data and see the value, then CR might have an issue...you have to 'read' the data to get the value, but there is still a way to get the value that you want....variables.
Instead of have CR do the running total, create your own. If the values are just to be seen inside the main report, global variables are fine to use in the formulas.
formula 1, set/reset values to 0, usually in a group header
globabl numbervar aTotal :=0
"" //hide the 0
formula 2, increment, since you are using SUM, I would suspect a group footer situation:
global numbervar aTotal := aTotal + SUM(SF1);
""//same reason...hide the output
formula 3, display, again usually in a group footer (the one that matches the reset formula...usually):
global numbervar aTotal;
HTH