The millions of calculations involved,had they been done by

原句应该是这种形式：
The millions of calculations involved,if they had been done by hand（这种虚拟语气比较容易看出来了吧?这里是插入语哦,对这个句子进行进一步的补充说明）,would have lost all practical value by the time they were finished.可以当成这样来理解
If they had been done by hand,the millions of calculations involved would have lost all practical value by the time they were finished.
虚拟语气中,如果从句时过去完成时,主句就用would have done的形式.