shortcode_atts() returns an array (which is assigned to $atts in your code).
It does not create new variables like extract() would – by the way using extract() is usually not recommended.
So in your code, $custom_field (line 11) is not defined.
The value you are looking for is in $atts['custom_field'] instead.
Changing line 11 to return bg_calculate_points( $atts['custom_field'] ); will fix it.