Armored_Sheep (BIS Dev): Specular map is a complex bitmap. Each color channel has its own purpose. You should understand well the way of calculating pixel colors in scene before you paint those textures.
As written in rvmat value of each pixel is calculated with RVMAT values and engine lighting values.
Texture _SM has diffuse values in RED, specular in GREEN and, Specular power in BLUE channel. _SMDI does not use RED chanel (better compression) GPU calculates those values automatically as Diffuse = 1 - Specular.
When you use specular map - diffuse, specular in RVMAT should be 1. Then paint B/W map with maximum specular and minimum specular values you imagine on surface. Than choose minimum (black) and maximum specular (white) values and change the bitmap levels to those values.
Solus: The way I did the specular maps is by taking the regular color texture and making the red and blue channels full white in Photoshop and lowered the brightness and contrast in the green channel. Basically the specular maps should have flat white in the red and blue channels, then the brightness of the green channel controls how shiny each area is, from no shining on the darkest part to a mirror like shine on white. I think the red and blue channels might also control something but most of the BIS specular maps have just white and it seems to work fine.