How deep do your females dig to bury their eggs?

How deep do your females dig to bury their eggs? I have be measuring the nest depth this year and this is what I have seen. In January I had four clutches. The top part of the nest was only 3 to 4 inches down with the bottom being around 6 inches down. Last night the bottom of the clutch was a little over 8 inches down and the top was about 6 inches down. Most of my females are about the same size, around 7.5 inches in length. Has anyone else kept track of how deep the going and does anyone know if they dig deeper nests when it is hotter out? Thanks Jonathan http://www.geocities.com/diamondbackterrapins