Tuna is great.
I don't think you have to go that low carb. It depends on your cardio and what body fat your level your aspiring to. It doesn't sound like your planning to hit the stage and would be pretty happy with mild ab definition.
Another point: many people here say that HIIT is catabolic or other steady state cardio.
True for a massive bodybuilder, as aerobics will results in aerobic adapation, and possible loss of muscle mass. But if your not particularly big - e.g., me - your not going to lose muscle mass running your arse off if you consume some carbs (my experience anyway). Moreover, I've found that I can maintain strength with carbs.
Judging by your (please don't take offense) lack of knowledge displayed in the post, I'm guessing your not a massive bodybuilder.
If that's the case, feel free to do cardio IMO.
Basically, my opinion is
(1) if ur doing mod to high intensity cardio, consider 30-40% C, 40% P, remainder fats
(2) if ur going to only do low intensity cardio, you could consider lower C.
But 40/40/20 is a nice diet to start, as it gives you room to move if your weight loss plateus. Consider buying "burn the fat, feed the muscle" - E'Book.
Also, have a look at a site like cron-o-meter, or fitday and track your diet
And haha about the guiness - guiness isn't certainly not on the cutting diet.