You can use the action addTabAfterinstead addTab, then you must specify the parameter <after>(categories, websites, upsell ...).
I edited your tutorial code, for example:
<reference name="product_tabs">
<action method="addTabAfter">
<name>custom</name>
<block>inchoo_customlinkedproducts/adminhtml_catalog_product_edit_tab</block>
<after>upsell</after>
</action>
</reference>
Hope this is enough for you.
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