这句话的最后的定语从句的先行词究竟是哪一个?

这句话的最后的定语从句的先行词究竟是哪一个?
James Boswell,his biographer described the garret where Johnson worked as 'fitted up like a counting house' with a long desk running down the middle at which the copying clerks would work standing up.
最后的at which the copying clerks would work standing up中which指的是哪一个?是 garret 还是 desk 还是 middle
我觉得一般从句子结构上说应该是middle ,而且有个词组at the middle of ,但是从意思上讲不通,人站在中间,桌子放在人站着的那个中间?
如果是desk,意思也不对啊,人不是站在桌子上.
如果是garret ,用介词 at at the garret?