How is the correct testosterone level determined for my body? i had blood work done and my doctor said its fine even tho its lower than average my age (20) but this worries me since i never experienced any penile growth during puberty. suggestions?

The "normal range" is defined using population studies of people at various ages with normal growth/development and function. The reported normal range can vary some from lab to lab based on the test used and their validation measures as determined by those who produce the test materials. Those who test above or below are correlated with those with known problems.