Such a scheme can be quite complicated if it takes incremental backups, multiple retention periods, and off-site storage into consideration.
This scheme has the advantage that it retains the longest possible tail of daily backups.
It is based on the mathematics of the Tower of Hanoi puzzle, using a recursive method to optimize the back-up cycle.
Many variations are possible, and the concepts are readily extended to disc-based directories containing backups.
Here are some options: Coverage automatically gets sparser the further back in time one goes, which approximates the likelihood of needing to do restores from past backups.
This technique probabilistically ensures that past generations are always distributed across all points in time as is desired.
The weighted random method only has an advantage over a more systematic approach, when backups are irregular or missed.
This method has the advantage of ensuring even media wear, but requires a schedule to be precalculated.