D3D alpha blending to a rendertarget surface

[Clootie]

If same formula applies to alpha channel then the resulting alpha chennel is:

A = ($7F*($7F/$FF))+($FF*(($FF-$7F)/$FF)) = $BF

Am I understanding this correctly, or am I not getting it at all?

You are 100% correct. In DX8 both color and alpha are scaled by the same blend factor.

PS. In DX9 there is D3DRS_SEPARATEALPHABLENDENABLE that allows separate blend factors and blend ops for alpha channel.