would anybody know how to calculate the area of these slotted holes? I wanted to make sure the slotted hole area combined are bigger than the cross section hole?

My quick method was to calculate the surface area of the body before the slotted cuts and then get the SA after the cuts have been made and take the difference of the two.

that will give me the difference in surface area which I believe will be accurate? then I can calculate the cross sectional area and compare the two? Would that work theoretically?