Finding entity by model is a poor way.
You could loop all grenade entities and check m_bIsC4 offset, then break the loop once you found it.
Or hook BombDrop event and use read_data to get CoordX, CoordY and CoordZ from the event. Just remember to use correct filters(4 th arg is 1 when planted and 0 when dropped), so register_event("BombDrop", "OnBombDrop", "a", "4=1").
Creating 32 task at 0.1 is not your best bet. Make only one at 1.0 seconds and loop players, or(as Arkshine told you) hook grenade think when bomb is planted(so on BombDrop event) and update the distance. Don't forget to remove the hook once it's no longer needed.