I am trying to do a very simple thing, But I think I lack some technique, so please help me out.

I have a shaft whose length is defined by a global variable called "length". I have holes on the shaft at different places

Case:1....... when the length of the shaft is 50 inches or less there should only be two holes on each end of the shaft.

Case;2.........when the lenght of the shaft is 50 to 70 inches there should be 2 holes on each side and 2 holes in the center.

case:3..........when the length of the shaft is 70 to 90 there should be holes on each end of the shaft and two holes on the 1/3 part and 2 holes on the 2/3rd part.(attachment is a picture of this)

Basically i want some method where i can put a range to the length to unsuppress or suppress a feature(in this case holes). I tried using if statement but it never took me anywhere. anyone to solve this little problem...