Lv.5 Manyu's should be used to upgrade girls between level 1-20.
Lv.20 Manyu's should be used to upgrade girls between level 20-40.
Lv.100 Manyu's should be used to upgrade girls above level 40.

If you use a Lv.5 Manyu on a high level girl, it will cost way more gold so definitely avoid that! I personally use Lv.5 Manyu's on girls up to level 30.

2* girls need 1 Lv.20 Bloom and 4 Lv.5 Bloom to evolve.
3* girls need 3 Lv.20 Bloom and 2 Lv.5 Bloom to evolve.
4* girls need 1 Lv.100 Bloom and 4 Lv.20 Bloom to evolve.
5* girls need 3 Lv.100 Bloom and 2 Lv.20 Bloom to evolve.

Don't ask me about 6* girls, I don't have any. :3