The overhead 30% is kept incase they try to go malicious and mess with the loan.. So yes, you can borrow up to 70% of the HP into liquid HIVE.. but if the one loaning tries to do something shady, then the account will be completely locked from the user and the 30% remaining will be taken as a penalty, going to the site / HLSHARE holders. It seems a bit overkill, but is meant as a large deterrent.