You can do this by using a "distribution shader V4" as blendshader.
Create one and connect it to the right input port of the strata shader.
Inside the strata shader choose "blend by shader".
This distribution shader now controls how much the strata is present.
If you decrease the "coverage" setting in the distribution shader from 1 to 0.5 you'll see that the "strength" of the strata will be reduced by 50%.
(you could even do this by using a "constant colour" node as blendshader and set that to anything <1, as this is exactly the same the distribution shader does when you reduce coverage...as you only lower the intensity of the blending colour)
Since the distribution shader also offers you control over slope and altitude you can also use those to control the strata appearance.
Be aware that you need to compute your terrain first to use those altitude/slope restrictions.
(strictly technical that would be "compute normal" for slope restrictions and "compute terrain" for slope + altitude, as a compute terrain also calculates texture coordinates which provide succeeding nodes of altitude information).
Hope this helps.
Cheers,
Martin
