How do we know the Raa rule?
First look at the vowel on the Raa itself; if it has a sukun, look at the vowel of the letter before it.
Heavy (tafkhim)
Raa is heavy when it has a fathah or dammah, or when it has a sukun and the letter before it has a fathah or dammah.
رَبَّنَا · قُرْآن
Light (tarqiq)
Raa is light when it has a kasrah, or when it has a sukun and the letter before it has an original kasrah.
رِزْق · فِرْعَوْن
Raa with sukun
When the Raa has a sukun, the rule depends entirely on the vowel of the letter right before it.
مِرْيَة · الْأَرْضWatch out: Some rare places (a Raa with sukun after a kasrah and followed by an elevated letter) have a special rule outside the general one; we leave them out of the basic games.