That's the tricky thing about maths - it tends to assume simple relationships. If the voltage goes up the current will go up and so will the power 'cos there's a fourth variable which doesn't obey simple rules. It's the bulb filament resistance.
Use 12v in the sums 'cos everyone does - I'm just being an awkward pedant. The value of current a fuse blows at is also variable, they'll happily carry more than their rated current but they'll pop on a smaller load 'cos things like bulbs and motors take a surge current when switched on.
The main reason for fusing in a car is to stop it bursting into flames if there's a short circuit so use a big enough fuse for the load (total current plus a bit) but a small enough one so that the fuse burns out before the wiring does!
