Alcohol consumption is down to time it takes for you to drink.
I drink loads as I have said but my first 3 cans is over a period of about 3 hours.
1.8 units x 3 = 5.4 units
The body processes roughly 1 unit an hour, so if I drink 3 cans over 3 hours I only have 2.4 units of alcohol in me.
Now if I drank 3 cans in an hour which is hard with Guinness then I would have nearly twice the amount of alcohol in my system at 4.4 units.
Time is the key with drink, drink too fast you will be mashed and dead pretty quick if you do it all the time.
If you sip a nice whiskey and it takes you an hour, then nearly half of the units are cancelled out.
For example say I have 10 pints over the course of 7 hours.........
10*2.2=22 units but as it has taken 7 hours to consume 10 pints then I only have 15 units inside my body.
I sleep a minimum 8 hours a day so there is another 8 units taken away, so am left with 7 when I awake. Although I am a little different as I have a very high metabolic rate.
When I was tested at the clinic after a serious bender with the lads I had about 14 pints, I came out at just under the limit for driving on only 6 hours sleep.
As I don't drink in the day normally and only usually drink around 5-6pm by that time my body is totally alcohol free.
Each to there own some people can and some cannot, I do not promote alcohol abuse as my cousin is doing it now. (just remembered go to ring my mother see you in about an hour)

Serious chats going around the family at the moment over her.
Point is if you worried you doing a bottle of whatever a week then stop.