How do I prepopulate a field with calculated options, when user is editing node?
Use hook_form_alter or hook_form_FORM_ID_alter to pre-populate your dropdown list with results returned from a custom function.



For example, using a form of hook_form_alter(): 

$form['field_my_field']['und']['#options'] = _my_custom_function();


Then make sure your function _my_custom_function() return's a proper array.

function _my_custom_function() {
    $custom_values = array();
    $custom_values['key1'] = 'value1';
    $custom_values['key2'] = 'value2';
    $custom_values['key3'] = 'value3';
    return $custom_values;
}


Obviously you can include some 'logic' in the _my_custom_function() to return your calculated list.



Here are a few super-useful link's I wish I would have know about before.


The Devel module has two functions that make hook_form_alter simple as pie... check out dpm() and dd().   
The FormAPI page is priceless for all things hook_form_alter().  
The Example Modules for Developers makes for an awesome intro to the FormAPI