Help getting a calculated field to work please?

Hi,

I need to add a field that returns an indication to the user, if a refund could be due when a customer cancels their order.

I have built out the logic but it fails to save every time. Have I made a syntax error or done this in the wrong way for Sugar Logic?

(I have tested separately both the lead_time and delay_days formulas and they both work fine returning the correct numbers)

This is the criteria for whether a refund is given or not:

lead_time = abs(subtract(daysUntil(related($pr_selected_vehicle,"pr_anticipated_delivery_date_first_update")), daysUntil($pr_date_procurement_signed_off_order)));
delay_days = abs(subtract(daysUntil(related($pr_selected_vehicle,"pr_anticipated_delivery_date_first_update")), daysUntil(related($pr_selected_vehicle,"pr_anticipated_delivery_date_new"))));
delay_tolerance = if(
    lead_time <= 28, 28,
    if(
        lead_time <= 56, 42,
        if(
            lead_time <= 84, 56,
            if(
                lead_time <= 112, 70,
                if(
                    lead_time <= 140, 84,
                    if(
                        lead_time <= 168, 98,
                        112
                    )
                )
            )
        )
    )
);
if(
    delay_days > delay_tolerance,
    "Refund Possible",
    "Refund Unlikely"
);



This is the error: