I make sure to always err on the side of more of the hardener
(black lid). That way it is sure to cure. If you err on the side of
less hardener it will sometimes never cure.
That's a mistake. The words "resin" and "hardener" are somewhat
arbitrary. Epoxies are two resins which must be mixed in proper
proportions. Too little or too much of either one will equally reduce
bond strength. Adding too much "hardener" is not better than too
little. Some epoxies, when the 50/50 ratio is not accurate, can loose
as much as half their bond strength if the mix is off by as little as
ten percent. Most consumer epoxies are formulated to be more
forgiving, but the principal is the same. Both componants are
equally essential for proper polymerization. It's not that one
componant somehow causes the other to harden. It's that the two need
to combine and react with each other.
You're best bet, if you want the most consistant joints, is to use a
small gram or carat scale, and actually weigh the two parts, when
you're using an epoxy type that doesn't meter both parts for you
(like the dual syringe types). Just adding an excess of the part
labeled "hardener" doesn't give you a better cure than having less.
If you find that in a given instance the epoxy isn't curing as fast
or as well as you'd like, add some gentle heat, such as from a heat
lamp, or an oven set very low.