For a scientific work for school I decided to measure the gravity constant with the Cavendish experiment.

I set up a structure like the one suggested on this website: http://www.school-for-champions.com/science/gravitation_cavendish_experiment.htm

I actually know there will be some inaccuracy, because I did not build a case for the experiment. It is standing in the basement of a house so vibrations have nearly no influence. The small masses are wrapped lead sheets, that weigh each about 120 g. The bigger masses are weighing 2 kg.

Today I measured all the needed values ($L, \theta, R_e, M$, like described on the site; $T$ was only measured quite inaccurate yet[+-50 secs possible])

My values are: $L=0.23 m, \theta = 7.44° = 0.13 rad, R_e = 0.09m, M=2kg, T=100s\pm50s$

But when inserting this into the equation I get an inaccuracy of 1000 to 10000 (depending on values for $T$):

$$G=\frac{2*\pi^2*L*\theta*R_e^2}{T^2*M}=2.4*10^{-7}$$

Where does this huge inaccuracy come from or how can I make the experiment more accurate (20% accuracy would be the best)?