Dfyjkt

Using both calc and |- would be a convenient way to refer to "inner, relative coordinates" of a node, with

• 
  (0, 0) meaning node.center,


• 
  (0, 1) meaning node.east and


• 
  (.3, -.5) meaning somewhere precise (but relative) in the south east quarter.

However, as described here and there, the following coordinate is not parsed:

($(node.center)!.3!(node.east)$ |- $(node.center)!-.5!(node.north)$)

So the following convenience macro does not work:

newcommandrelativeToNode[3]- $(#1.center)!#3!(#1.north)$)

How nice would it be to write:

node (inner) at (relativeToNodeouter.3-.5) hey!;

Is there any chance to get this working?

Any workaround that would not involve defining intermediate macros or coordinates (whose names would have to be picked etc.)?

For your use case you don't even need the calc library. You can define style relative to node=<node name> that shifts and scales the coordinate system accordingly:

tikzset
 relative to node/.style=
 shift=(#1.center),
 x=(#1.east),
 y=(#1.north),
 

Then you can place your node with

path[relative to node=outer] (-0.4,-0.5) node Hello;

MWE:

documentclass[tikz,margin=2mm]standalone

tikzset
 relative to node/.style=
 shift=(#1.center),
 x=(#1.east),
 y=(#1.north),
 


begindocument
 begintikzpicture
 node[minimum width=2cm,minimum height=3cm,draw=red] (outer) at (3,3) ;

 % Only for the grid
 beginscope[relative to node=outer]
 foreach ratio in -1,-0.8,...,1
 draw[help lines] (-1,ratio) -- (1,ratio);
 draw[help lines] (ratio,-1) -- (ratio,1);
 
 draw (-1,0) -- (1,0);
 draw (0,-1) -- (0,1);
 endscope

 path[relative to node=outer] (-0.4,-0.5) node Hello;

 endtikzpicture
enddocument

Resulting in