How can I recalculate field values of a rendered node? For example on a recipe node, exposing a "yield" field on a rendered node to recalculate ingredient quantity values
I dont think there is any module that you can use, you can try hook_node_view_alter hook to change the rendered.


  If the module wishes to act on the rendered HTML of the node rather
  than the structured content array, it may use this hook to add a
  post_render callback. Alternatively, it could also implement hook_preprocess_node(). See drupal_render() and theme() documentation
  respectively for details.


Check hook_entity_view_alter too

If you want to change in the node create form, there is Conditional Fields Module.


  Define dependencies between fields based on their states and values.
  Conditional Fields for Drupal 7 is an user interface to the new States
  API, plus the ability to modify fields appearance and behavior on
  certain conditions when viewing content.


Installation and Usage