I know I must keep ahead of the bees by adding on supers. The forum posts talk about adding when supers get approx. 70% full.
That is some misunderstanding. If you have 70% capped box, you should be ready to exctract it.
You may have 3 whole empty super on the hive and after one week they are all full of honey and capped. That happens on rape field.
You should evaluate how much is going to come and then give free combs for honey.
If you have one box capped honey, you have needed to more box where nectar have dryed up.
If you take 4 box capped honey in one time and you give 4 empty box on hive, bees spread nectar on whole comb area. If honey flow stops, all honey stays open. First of all:
You must give room for bees. Then if it occur honeyflow, bees fill hive with honey. If flow continues, and you take capped honey away, bees fill combs again.
Everybody has different kind of hives and different kind of pastures.
One hive site may give 3 times more honey tahn another.
70% rule is imposible to apply at least to me.