genomic DNA shearing with Covaris 12*12 tubes

[nfourier]

Hello, I am calibrating a new protocol and I need to shear my cross-linked DNA (2*10^7 RAW cell line) to an average size of 1000 bps. I'm using Covaris E220 with12*12 tubes. I did a time course with cycles per burst 175/200 and 5% duty cycles, 4,6 &8 minutes. The shearing is either to big (>2000) or too small (<700).
I'm running out of ideas. Any help will be greatly appriciated.
I'm using Ethanomics buffers and Covaris protocol. Unfortunately ther's no information for the 12*12 tubes only the microtubes.