It is important to consider the texture of the hair before perming.
There is 3 types of hair texture:
Fine hair will curl more easily, so you will need to use a weaker perm lotion.
Use a normal strength perm lotion for medium texture hair.
If the hair is coarser, it can be more difficult to curl. You will need a stronger perm lotion.
Hair texture is also the same as hair thickness.