weirdness with transparency

Hi all

probrally an easy question for many but one thing I can't get my head around is transparency, I have looked at many docs but am getting nowhere (again this is more a GL question than a openTK one).

I have a menu that is made up of 5 texture mapped polygons that move in the style of the win tab switcher in compiz fusion.

I have enabled transparency like this

GL.Enable(EnableCap.Blend);
GL.BlendFunc(BlendingFactorSrc.OneMinusConstantColorExt , BlendingFactorDest.One);

and while it does make the polgons transparent, the problem is that they are extreamly transparent, and i only want about 80% opacity.

I gather that it works from the alpha channel of the texture but as far as i can see, if that were true they should shouldnt be transparent at all.

A slight hint would be appreciated, as I am confused

thanks
Rob