What is proper ratio of sand to portland cement to use for repairing a 30 yr old stone wall
Since you asked it's 6 parts sand/1 part cement/1 part hydrated lime; it won't stick without the lime. Unless you absolutely need to use a specific sand you're much better off buying a bag of premixed mortar. You can take a piece of the existing mortar to a masonry yard and get a color matched dye if you need an invisible repair. 

A helpful pneumonic device for mortar is "MaSoN wOrK." The capital letters represent the standard mixes in decreasing order of compressive strength.


Type M has the most cement and is used almost exclusively below grade.
Type S has the highest tensile strength and is used for outdoor load-bearing work such as retaining walls.
Type N, the 6:1:1 mix above, is used indoors and out for soft stone and brick work.
Type O is limited to non-bearing use indoors.
Type K isn't a thing anymore, outside of restorations. As you get to this end of the spectrum there's less cement and more lime so things are a sticky mess.


You'll find bags of Type S and Type N in the home center. You can use the same mixture all over the wall. I'd suggest N based on your description.