I can speak about the commuter flavor of bike (but know nothing about the racing flavor). You should have a little bit of clearance when your feet are on the ground and you straddle the frame. The shoes you normally wear when you bike might play a small role in the frame size you chose. I wear street shoes when I bike, so when I pick out my frame size, I always wear street shoes. After buying a handful of bikes in the last eight years, I now know what my frame size is ... or least where to start looking. One more thing, the size of the tire might play a role as well, but a salesman should get you in the 'neighborhood'.
My inseam is 33" and I've bought bikes with a frame size of 59cm (a bit too big) and 57cm (almost perfect). I would like to have a 56cm frame, but the brand I buy (Batavus), doesn't make that frame size in the models I want to buy.
The next smaller size in the model I bought was 53cm and while vertically that felt OK (with the saddle properly adjusted), the handelbars were much to close ... things were out of proportion; I would have looked like an adult riding my young son's bike.
After you zero in on the frame size, adjust the saddle so your legs are almost , but not quite straight, when you pedal. You should have a break in your leg. I don't know what the 'degree' measurement is, but a good bike saleman should get you in the ballpark settings. I always play around with the handlebars and saddle settings until things seem right. If the saddle setting is too high you can inflame your Achilles tendon. I once biked a long distance when my sadlle was too high and it was very cold outside. I didn't have my allen wrench with me and just persevered; I knew the saddle was too high, but I never thought I'd hurt myself. Well, I wound up with extra fluid in my Achilles tendon ... it made a crunchy sound when I moved and I was out of action for a while. I learned a lesson.
I hope this has been of some help and if I've given you bad advice, I hope someone points that out. I am not an expert ... I've just bought a few bikes over the years and asked a lot of questions in the process.