The average membrane diameter per circle is (3.2+4)/2=3.6cm.

The average circumference of the membrane is 3.6 * 3.14 =11.304 cm.

The total thickness of the membrane is 4-3.2 = 0.8 cm.

The negative cubic of the film thickness of 0.8/(6000/11.304) =1.5072 *10.