Six layers of 3mm glass that have been previously fused together would be less than 2 cm of glass (in thickness). So, I looked for one of our published fusing cycles that involved heating up a piece of that thickness to less than a full fuse temperature. I found one under the "Learn More" tab over the "Harnessing Glass Flow" project. The slumping cycle there suggests heating the slab quite slowly compared to the first full firing. After holding for 20 minutes at 1000° F to allow the heat to be distributed as evenly as possible, you can go up to the process temperature (anywhere from 1180-1250°F, depending on your kiln). Hold for 10 minutes there. Then anneal according to our annealing schedule for thick slabs. If your frits on the top level are of various thicknesses, you will want to anneal more slowly than we recommend for a slab of even thickness.
For the details, you can find the firing cycle here: https://videos.bullseyeglass.com/wp-content/uploads/2018/07/Harnessing_Flow_in_Kiln-Glass_-_Learn_More_L048.pdf
Watch the "Harnessing Glass Flow" video for an explanation of why the glass needs to be heated so much more slowly than it did on the first, full firing.
Does that help? If you are uncertain, you could make a smaller model of the project in the same thickness, maybe 3-4 inches in diameter, and take that through both cycles (first the full firing, and then the tack firing) so you can personally observe the results before you work on your intended project.
PS: If anyone has other suggestions, chime in. I haven't actually done a project quite like Charlotte's, so others may have good advice based on personal experience.