Bear in mind that if a particular school is your closest school, you get priority over people for whom it's not their closest school. Rule 5 is: closest school (with distance used to determine who gets in, if more people apply under that rule than there are places) and rule 6 is just distance from school. So if you don't get into your closest school (ie rule 5), you might not get into your second closest school (applying under rule 6), if that school has filled its places with applicants for whom it's their closest school. Does that make sense?
For example, this year my second-closest school (let's call it School B) filled all its places with siblings and people for whom it was their closest school. So we had no chance of getting a place as it's not our closest school.
However, School B is the closest school to my friend, who lives down the street from me. So she would apply under rule 5, but this year wouldn't have got in, although it's her nearest school, because she lives 600m away and the farthest applicant lived 400m away this year. On this year's data, she wouldn't have got into her second closest school (let's call it School A), because she is too far from that, and would be applying under rule 6, so everyone for whom School A is their closest school would get in before her.
The www.hertsdirect.org.uk/admissions site has a tool where you put in your postcode and it tells you the 6 or so nearest schools to you, and gives you the exact meter-age. That's the figure which they'd use in admissions.
Also, just to say that schools in Herts don't have catchment areas as such - it is all done on distance, so some years (eg with lots of siblings), you might have to live, say, 300m from a school to get in, and other years (eg fewer applicants) you might have to live, say, 400m from the same school to get in.
I should say that all this advice relates to non-faith schools - I don't know much about them as it wasn't relevant to me, and they have their own admissions criteria.