All skills start at the base percentage, plus any O.C.C. or IQ bonuses. The percent per level is the increment per additional level of experience (as in, beyond level 1). Here is a quick explanation of how I view the different skill categories (O.C.C., O.C.C. Related, and Secondary).

O.C.C. Skills: These skills are those that all people of a particular trade (O.C.C.) learn. It is imperative that a character know these skills in order to carry out tasks common to their occupation.

O.C.C. Related Skills: These are skills that aid the O.C.C. category. They represent the slight differences in background among the same occupations.

Secondary Skills: These skills are considered to be background skills (most people consider these "freebie" skills to take as Weapon Proficiencies and bonus providing skills rather than use these to build a character's background). This means that a character who grew up in a family of fisherman or near a coast or lake is likely to have the Fishing skill. Someone who grew up in a family of entertainers is likely to have Sing, Dance, or Play Musical Instrument. It all depends on the character's background. Because it is so dependent, I allow a person to take Secondary Skills from any category, regardless of what the O.C.C. specifies. I only require that the skills be worked into the background in some way. All secondary skills are still limited in that they do not receive any bonuses (further reflecting the background nature).