It goes like this:
45 dev is five base list three levels.
Next level, five other base list three levels is 45 additional dev.
Next level, 60 dev is a mix of five additional open or closed list three levels.
So at third level, the mage has 15 list to level 3.
Keeping this pattern of development, the mage will have 15 list to his level.
At level 21 our mage has 15 list to level 21. Well, probably 10 base to 21 and any closed list will stop being developed at that point due to cost, while the open may or may not continue. At this point, the mage may wish to develop new list.
A friendly GM might allow a level 20 PC to develop a new list the allowed 15 levels in one level at 4/4/4, this being roughly the same as five list at cost. Some GM's might want to use skills such as teaching for intensive learning, time requirements, etc. This would allow the mage at each level of advancement to learn a new spell list available 15 levels, mastering to his level a list in two levels of advancement. If allowed, a restriction of one list being uber advanced a level sounds reasonable...unless you think high level sell users should be able to qickly learn list. HARP has simular rules I understand.