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Please review the calculation in the appended pdf file. In the general case, the output current isn't independent of output voltage, in so far your schematic is incomplete. You usually try to implement a current source with (ideally) infinite output resistance, the conditions are derived in the document.
It assumes however infinite OP gain, introducing finite OP gain further increases the complexity.
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There are also various previous threads about the Howland current source, e.g. https://www.edaboard.com/thread317636.html