In case anyone finds this (like me), things seem to have gotten a lot simpler. The process I used to get get user-specific data saved for my WordPress site is:
post=, this is the field group #.
The code I used in the custom form:
acf_form(array( 'field_groups' => array(123), //use the field group for your form 'post_id' => 'user_'.get_current_user_id(), ));
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