Possibly like this:
if (
Sum ({AgentProductivity_Element.DUR_INC}, {AgentProductivity_Element.EXT_NUM}) +
Sum ({AgentProductivity_Element.DUR_SIGNON}, {AgentProductivity_Element.EXT_NUM})
) = 0 then 0
else
(
Sum ({AgentProductivity_Element.DUR_INC}, {AgentProductivity_Element.EXT_NUM})) %
(
Sum ({AgentProductivity_Element.DUR_INC}, {AgentProductivity_Element.EXT_NUM}) +
Sum ({AgentProductivity_Element.DUR_OUT}, {AgentProductivity_Element.EXT_NUM}) +
Sum ({AgentProductivity_Element.DUR_SIGNON}, {AgentProductivity_Element.EXT_NUM})
)