Hey Pol,
Capt. Marvellous is right. The answer to your question is: no limit.
Regarding the 30 psi etc, that is when you do a crossbleed start.
You can find this in vol. 1 B737NG page SP 7.3. The 0,5 psi I can't find either in the B737NG. In the 'classic' it is written.
Good luck