You should have sufficient service. Even if every spot is taken on your panel, most homes use less than 40amps. I would not sub off of a sub. The wiring gets too complicated. Your best bet is to pull from the main panel. If there are no spaces, move a few circuits to the first sub panel and then use the open spaces to supply the second sub panel. IMO this would be the best solution. Load calculators can be found on the net. I don't have enough posts to insert a link, so just google online electrical load calculator.
OK thanks. Im pretty sure I have plenty of service. Just no room to pull from the main.