If check box is check minus field amount from total

Hi There,

I need some help with an if statement in my field.

Currently my [rate total] field is a sum of many fields added all up as below:

Sum([Cost - Total],[Markup],,[Stuffing Cost],[Cost - GRI])

I currently have a check box that I want a user to click if they need to subtract a specific amount in another field from the total

If [CT LSS collect] = true

I need the system to subtract the amount in field [CT LSS] from the total.