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This is the existing calculation - I hope (used in both debug on the TX and in the RX) which appears to read high

// --- ADC1 is data from my UV sensor UVM-30A
  const byte UV_Transmission = 100;
  IndiceUV = ((avg_IndiceUV * 12.0 * (100.0 / UV_Transmission)) / 1024.0) ;

If I change the *12.0 value to *9.0 then the IndiceUV values fit almost perfectly with the UVM30A data sheet scale

// --- ADC1 is data from my UV sensor UVM-30A
  const byte UV_Transmission = 100;
  IndiceUV = ((avg_IndiceUV * 9.0 * (100.0 / UV_Transmission)) / 1024.0) ;

As an example 700*9*1/1024 = 6.1 - near perfect

Debug data with the above change - again pretty much bang on

Sending ID4...
ACD0 - SolarRad :545   SolarRad :745
ADC1 - avg UV   : 937  Indice UV : 8.2
 
Sending ID4...
ACD0 - SolarRad :547   SolarRad :748
ADC1 - avg UV   : 950  Indice UV : 8.3
 
Sending ID4...
ACD0 - SolarRad :548   SolarRad :749
ADC1 - avg UV   : 954  Indice UV : 8.4
 
Sending ID4...
ACD0 - SolarRad :549   SolarRad :751
ADC1 - avg UV   : 939  Indice UV : 8.3
 
Sending ID4...
ACD0 - SolarRad :550   SolarRad :752
ADC1 - avg UV   : 934  Indice UV : 8.2
 
Sending ID4...
ACD0 - SolarRad :553   SolarRad :756
ADC1 - avg UV   : 935  Indice UV : 8.2
 
Sending ID4...
ACD0 - SolarRad :555   SolarRad :759
ADC1 - avg UV   : 934  Indice UV : 8.2
 
Sending ID4...
ACD0 - SolarRad :557   SolarRad :762
ADC1 - avg UV   : 955  Indice UV : 8.4