Conditional formatting for hours calculations

Hi…

I need help for applying conditional formatting to a cell for calculating hours [h]:mm by comparing 2 cell , one has a dynamics value (changeable) & other is static (locked)…

The scenario:
Current Generator Hours = 861:35
Generator Servicing Hours = 1000:00
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Conditional formatting state using icons:
Case1 (Green Icon) : before 336 hours from generator Servicing Hours
Case2 (Yellow Icon) : before 120 hours from generator Servicing Hours
Case3 (Red Icon) : before 72 hours from generator Servicing Hours
Case4 (Black Icon) : when it reach servicing hours & afterward
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I hope my Q is clear...

Thanks & Regards…