I am the kind of person that 'knows enough to be dangerous' so I am trying to make a change to a VF page and having some difficulty with it. We use one VF page for New and Edit buttons for a custom object. Currently, we have a field 'Tax Group" that is read only on creation but editable after the record has been created. I'd like to change the 'Tax Group' field that displays on creation to a formula field from another object. A 2nd but less preferable solution is to hide the field from view with creating the record. Formula field (percent field type) is straightfoward - Object__r.Tax_Percent__c. This is what the section of that page currently looks like.

        <apex:pageBlockSection columns="2" title="Purchase Order Changes" collapsible="false"> 
                    <apex:inputField value="{!Con_Req__c.Tax_Rate__c}" rendered="{!taxExempt == false}"/>
                    <apex:outputField value="{!Con_Req__c.Tax_Rate__c}" rendered="{!taxExempt}"/>                         
                </apex:pageBlockSection> 

I've tried changing the first line under the page block section to this, hoping it was a simple tweak:
<apex:inputField value="{!Con_Req__c.Tax_Rate__c}" rendered="{!taxExempt == !Object__r.Tax_Percent__c}"/> but I get an error -
Unknown property 'Con_Req__cStandardController.Object__r'

This is the first line on the VF Page with the controller statement.
<apex:page standardController="Con_Req__c" extensions="con_req" action="{!setProj}">

Is anyone able to helpwith this? Again, being able to hide the field is an option if displaying the formula field is not.

Many thanks!