You're correct, an average of 70 or more across 5 observations will e a 'pass' on that criteria. If you also pass the other criteria (fitness test, workshop/LOTG, eligible matches) then that will enable your County FA to nominate you to the FA for promotion to Level 4.
However whether 73 turns out to be a "good" mark will depend on two further things. Firstly, how that stacks up against your county colleagues attempting the same promotion, as your nominations to the FA will be ranked from most promotable to least promotable. Secondly how many new L4's the FA need in your area ... which is very hard to estimate.
As such, you just need to average as high as you possibly can and then keep your fingers firmly crossed