I was playing around with a macro to keep Light's Grace up on me today. In general, Flash of Light is a superior spell to downranked Holy Lights. If you compare a downranked Holy Light to a Flash of Light with equivalent mana consumption (for example, HL4 vs FoL6), you'll see that the Flash of Light has a higher base heal-per-second and--more importantly--a higher coefficient-per-second.
(Coefficient-per-second refers to the amount of +healing from your gear that is applied per second. If a spell has a higher coefficient, it means that there is always a point where it will heal for more than a spell with a lower coefficient.)
However, a downranked Holy Light does have the advantage that it will give you the Light's Grace buff, which can be invaluable when you need to throw a big heal quickly. A two second HL11 can be the difference between a dead tank and a boss kill.
So I decided to try out a macro that casts Flash of Light most of the time, but casts a downranked Holy Light just enough to keep Light's Grace up:
/castsequence reset=10 Holy Light(Rank 5), Flash of Light, Flash of Light, Flash of Light, Flash of Light, Flash of Light
What this does is cast HL5, 5xFoL7, and then repeat. You can play with the number of FoLs and the reset timer to ensure that Light's Grace never drops off. I personally don't constantly spam, so I have the HL5 coming a bit earlier than necessary. You could add a couple of FoLs to squeeze out the maximum effect.
I tried out on Gruul, and it seemed to work reasonably well. It felt a little bit easier to nail the OT with a 2s HL11 right after coming out of a Silence. The majority of our healers are paladins, so the Hateful Strike, Silence, Hateful Strike sequence is a killer near the end of the fight.
 Technically, if you bring Blessing of Light into the picture, FoL6 has a lower base heal-per-second than HL4. But FoL6 becomes strictly better at about +900 healing, which is relatively trivial for a raid healer to achieve.