I also have the Equal-i-zer brand hitch, but mine has the adjustable hitch head on it. On the ones with the adjustable hitch head, the bar tension is increased by "leaning" the hitch head more toward the rear of the vehicle. The L-brackets (on the trailer A-frame) are only adjusted to make the bars level to the trailer frame.
However, since you say you do not have an adjustable hitch head, it seems to me that your only choice for increasing the tension on the bars would be to bump the L-brackets up a notch.
As for your question of when to stop adjusting- I stopped adjusting mine when the truck sat level when it was loaded down like I was about to head out. And the bars are springs- they are supposed to bend when driving through a dip in the road.
2005 27' Coachmen 241 FKG
Pulled by 2004 Ford F-350 SRW PSD