Basically how this plays out is, how much of my healing do I need to do with unglyphed frenzied regen so that I know ovreall I'm getting more healing that if the healers where getting 40% extra healing on me.
total healing = t, FR healing percent = p
t*p = (t - t*p)* .4
p = .4 - .4p
1.4p = .4
p = .4/1.4
p = 28.57%
This works out to 28.57%. So if I do 28.57% of my own healing, or more, unglyphed is more total healing on me. If I am doing less than I would get more healing from healers getting 40% more.
I've extended it a bit further on my spreadsheet to account for uptime on glyphed FR base on your rage per second you generate.
As you can see because currently I only have 9.33 RPS I can only keep it up 93.33% of the time. so this lowers the benefit a bit on glyphed where I only need to be doing 26.67% of my total heals.
Keep in mind, this is just the math! You should also consider things like you being an extra "healer" and the benefit of that. Things like fight or 10 vs 25. Play smart, play bear.