Paint.NET :: How To Calculate Resulting Color Alpha

It's not really related to Paint.NET usage, but how it calculate the resulting color:
 
For example I make red background, call it C1:
 
RGBA = 255, 0, 0, 255
 
Then I place a yellow pixel, call it C2:
 
RGBA: 220, 216, 0, 200
 
Then I use the Color Picker tool on this pixel, I get C3:
 
RGBA: 227, 169, 0, 255 
 
I already know how to calculate R, G, and B of C3
 
R = ( (C2.A / 255) * C2.R ) + ( (1 - (C2.A / 255)) * C1.R ) = 227
G = ( (C2.A / 255) * C2.G ) + ( (1 - (C2.A / 255)) * C1.G ) = 169
B = ( (C2.A / 255) * C2.B ) + ( (1 - (C2.A / 255)) * C1.B ) = 0 
 
But for the Alpha channel, it is different and I can't find how to calculate it!