Ok so I was reading this and nobody seems to have said a definate how the latent kicks in.
First off I'm lvl 51 with HQ staffs and austers gear that being said ramuh costs me 3 a tick. I put the ring on no effect. went and got some bats to beat on me a lil bit till I hit yellow hp. I did not wear any HP+ or - gear so what HP I had was my base. When I hit yellow HP I killed bats and brought out ramuh at 2 a tick. So mystery solved yellow HP unless you have HP effecting gears.
Which kinda sucks since well most smn/whm and /whm at 50 when you can wear the ring gives you HP regen.