Blending Cyan with an RGB image

I'm combining four 16-bit grayscale images into a 64-bit RGBA image. Each grayscale image holds data for one each of these color channels: red, green, blue, cyan.

I'm replicating an image program which merges red, green, blue and cyan channels and I'm not sure how they add cyan.

Should I calculate the addition of the cyan channel as follows?

R = r
G = g + 0.5cyan
B = b + 0.5cyan