As a holy priest:
I wouldn't recommend using ProM for that. When I was first doing it, I tried it a few times, but too many deaths later, I decided it just wasn't worth it. Plus, in the event that multiple people get tombed, it won't as much good (yes, multiple people shouldn't be getting tombed, but it happens, especially towards the end).
I use a combination of DMB and CoH. DBM is a must-have, especially for its /range feature which helps make darn sure you aren't going to get chained on to. I save CoH exclusively for tomb. In 25m, you are going to have 5 or 6 healers. KT doesn't do that much damage, and throwing ProM around every time it's up does most of your healing. I am generally not on the top of the meters for that fight, as I use ProM and renew almost exclusively, but we rarely wipe, so it's worth it.
When someone gets tombed, DBM puts the skull raid icon on the person. Do watch your raid warnings, as it will also do this for Detonate Mana. Typically, but not all the time, it goes Detonate Mana, then Tomb, back and forth. Don't rely totally on that, but it's a decent rule of thumb. I have yet to see him tomb twice in a row, although it could probably happen.
The second I see a skull pop up, and verify it's a tomb, I CoH the skull person. I have the holy reach talent, as I rarely have to MT heal, and I was finding CoH was not hitting all 5 possible targets in fights where it matters (KT, Saph, to name 2 off the top of my head). Invest in some crit food as well. Generally the CoH will crit on at least one of the targets and give you a free insta-FH. These 2 in rapid succession will save the tomb target. If the tomb chained to multiple people, the CoH will generally also save them as well, especially after the coming patch.
It's a stressful fight sometimes, good luck!