I may have linked to the following site for you already, but just in case: http://www.ephotozine.com/article/as...negatives-4682
That site isn't perfect, at least partially because it is hard to accurately portray the information through the internet, when everyone's computer screen is different, but like the site linked to in Bruce Robbins' post, it should give you an idea how best to approach the question.
One thing I would emphasize, however, is that underexposure can be terrible, whereas underdevelopment can often be compensated for.
Over-exposure and over-development make it more difficult to get good results, but unless the errors are very large, they too can be mostly compensated for.
Do you have good prints from your "professionally" developed negatives? If so, that will tell you something about them.