Now Look at the Vowel on Heavy Noon Itself

Structural Rules of Heavy and Light Noon-e-Ta'keed

Do not confuse the vowel before Heavy Noon with the vowel on Heavy Noon itself.
The source gives this final rule:

Vowel on Heavy Noon itselfFormsCondition
Kasrah6an Alif comes before Heavy Noon
Fathah8all remaining positions

Examples:
لَيَفْعَلَانِّ
لَيَفْعَلَنَّ
Two maps to remember: Heavy Noon enters 14; Light Noon enters 8.