Additional Feature Added To Estimating Takeoff Program

I have an estimating program that i would like to add additional features [login to view URL] program is in C# and .Net. Basically the progam measures m2 and lineal metres, what i would like to do is add a feature so that when i am measuring m2 or l/m on the flat i have an option to insert the degree of the pitch of the roof. The calculation will then multiply the m2 by the predetermined value to give the actual m2 of the roof on the pitch Say roof = 200m2 on the flat but is actually a 26 degree pitch roof then it would calculate 200m2 x 1.113 = 222.6m2 The same principle would apply to lineal metres See attached file showing values