I've been trying many things like modifying a slope, and other math functions but I cannot wrap my head around how to do it in Excel.
Did any of the math functions work for you, and you are not sure how to program the modified function into Excel, or are you still trying to figure out how to modify the function? If the former, explain to us your modified function, and we'll be able to help you program that into Excel. If the latter, we will probably need a better description of what you currently have and exactly what modification(s) need to be made.