How to define relative paths in Visual Studio Project?

Instead of using relative paths, you could also use the predefined macros of VS to achieve this.

$(ProjectDir) points to the directory of your .vcproj file, $(SolutionDir) is the directory of the .sln file.

You get a list of available macros when opening a project, go to
Properties → Configuration Properties → C/C++ → General
and hit the three dots:

In the upcoming dialog, hit Macros to see the macros that are predefined by the Studio (consult MSDN for their meaning):

You can use the Macros by typing $(MACRO_NAME) (note the $ and the round brackets).

By default, all paths you define will be relative. The question is: relative to what? There are several options:

1. Specifying a file or a path with nothing before it. For example: "mylib.lib". In that case, the file will be searched at the Output Directory.
2. If you add "..\", the path will be calculated from the actual path where the .sln file resides.

Please note that following a macro such as $(SolutionDir) there is no need to add a backward slash "\". Just use $(SolutionDir)mylibdir\mylib.lib. In case you just can't get it to work, open the project file externally from Notepad and check it.

If I get you right, you need ..\..\src

I have used a syntax like this before:

$(ProjectDir)..\headers

or

..\headers

As other have pointed out, the starting directory is the one your project file is in(vcproj or vcxproj), not where your main code is located.