Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih ibn ‘Uthaymeen, may Allaah preserve him, who answered as follows:
He should put the back legs of the chair in line with the other worshippers’ feet.
But then when he stands, he will be in front of the others
He isn’t standing, (so he should put) his back in line with the feet of the other worshippers. And Allaah knows best.