mortar for limestone blocks

Quoted text begins.Most of these objectives can also be obtained with the right
combinations of sand, lime and portland cement (masonry mortar) but
hydraulic lime has the advantage of allowing a thin, grout-like mixture
to do this, whereas a mix based on cement could require a lot of sand to
get the same reduction in hardness.
End of quote.

John,

What are the right combinations of sand, lime and portland? I know it
depends on the project and the stone itself. Ian Cramb in the Art of the
Stonemason recommends 7 parts sand: 1 lime: 1 portland. To me that seems
like a hell of a lot of sand. My boss on the other hand has the "strong
mortar's always best" unthinking prejudice: type S with extra portland
added. That blind adherance to strong mortar is even in the local building
code now - all exterior dwelling masonry walls must be type S.

I build mostly random rubble walls and stone furniture using stone from
around here (Shenandoah Valley, Virginia): fairly hard sandstone (lots of
quartz in it) and medium-hard but quite brittle limestone. As a mortar
expert, what mix do you recommend?