Configuring partman in preseed to configure disk partitioning?

Question

ubuntu

Interested in the question of setting up preseed on ubuntu to set the partition of the disk into partitions with pre-known parameters.
auto mode is not suitable, a test installation followed by "debconf-get-selections --installer > preseed.conf" produced settings that were not entirely clear (about 230 lines for three sections).
We are looking for a sensible man on this case :)
Thanks in advance.