OK, I found the relevant quote online. It was in Master and Commander, not Post Captain.
I'm not positive my interpretation is right, but it looks to me like "allowance" refers to what you get twice a day, and "at twice" means "in two lots" not "twice". And that both people agree on the figures. I wouldn't have asked if it didn't look fairly incontrovertable, else I would just have assumed I (or the author, or the character) had muddled the figures somehow which didn't seem unlikely.
How much did he drink? Why, now, Tom was a popular young chap, so I dare say he had the whole allowance, bating maybe a sip or two just to moisten their victuals. That would make it close on a quart.'
'A quart. Well, it is a great deal: but I am surprised it should kill a man. At an admixture
of three to one, that amounts to six ounces or so – inebriating, but scarcely lethal.'
'Lord, Doctor,' said the gunner, looking at him with affectionate pity, 'that ain't the mixture.
That's the rum.'
'A quart of rum? Of neat rum?' cried Stephen.
'That's right, sir. Each man has his half-pint a day, at twice, so that makes a quart for each mess for dinner and for supper: and that is what the water is added to. Oh dear me,' he said, laughing gently and patting the poor corpse on the deck between them, 'if they was only to get half a pint of three-water grog we should soon have a bloody mutiny on our hands. And quite right, too.'
'Half a pint of spirits a day for every man?' said Stephen, flushing with anger.
I'm not positive my interpretation is right, but it looks to me like "allowance" refers to what you get twice a day, and "at twice" means "in two lots" not "twice". And that both people agree on the figures. I wouldn't have asked if it didn't look fairly incontrovertable, else I would just have assumed I (or the author, or the character) had muddled the figures somehow which didn't seem unlikely.