Rotor equilibration without counterbalance
This is relatively obvious method, which I found not used by most of the researchers. Usually the equilibration of 2,3,4,6,8,9, and 10 tubes in 12x rotor of table top centrifuge is simple, but for 5 or 7 most of the people I know use an additional tube. This is not necessary. If you look at the figure provided here, you'll see that 5 is a combination of equilibrated 3 (red) and equilibrated 2 (green or blue). In this situation the rotor is equilibrated. Add two more tubers for 7. All the best in your research.