Write your opinions on the following options:

4 armies attack a land with a fort and 2 armies. Safe or not ?
5 armies attack a land with a fort and 3 armies. Safe or not ?
6 armies attack a land with a fort and 3 armies. Safe or not ?
7 armies attack a land with a fort and 4 armies. Safe or not ?
9 armies attack a land with a fort and 5 armies. Safe or not ?

Since all are possible but all can go wrong (and when I attack with the examples you mention its most likely to go wrong) the logical answer must be

4v2 not safe
5v3 not safe
6v3 not safe
7v3 not safe
9v5 not safe

I must agree in DirtyHarry

It depends on how much you need the fort and what the consequences are if you fail to take it.When I'm winning (which I obviously always am) and your oponent only has his capital country left, all those options are safe to do.

lol, when it comes to attacking to the cap I try even 1vs10, who knows what the probability will be :)

With neutral lands usually I attack with 6 in a 2 fort and with 9 in a 3 fort. Above I just leave them and take them in a walk through with more troops.

But if a land is strategically important or at war I don´t stick to that rule and try even lucky shots with 1 (for example in front of foreign caps)

4v2 no
5v3 no
6v3 yes
7v3 maybe
9v5 probably
why i would rather attack with 6 than 7?-doh u can loose only 6.
why do you have more chance 9-5 than 4-2? don't know its just true.

I agree with harry

Another way to frame the question:

If both you and your opponent are bordering a neutral land with a fort and some armies, would you like to attack first or not, considering you both have an equal amount of armies there?

e.g., 3 armies in fort. You and your opponent both have 9 armies bordering. Would you go first move?

In this case, I think I'd let my opponent go first, as I'd expect him to have about 3 armies left, which I would then be able to kill with a late move. If he'd have 4 left, chances would be about equal, 9-4, I think. But as we all know, forts are unpredictable, and he may get it with 6 or more.

Let's assume there is backup to reinforce the fort, so getting it is an advantage compared to not attacking.

In this case, I'd want to go first:
3-1
6-2
10-3
13-4
16-5

Purely based on intuition.

I prefer to go last or wait a turn. I try to take as few risks as possible, but sometimes you just have to go for it.

if u want to feel safe, make it this way:

4vs1
7vs2
10vs3

It's just a sliding scale. Sometimes you attack a fort 4v3 and get it, sometimes 6v1 and don't. Sometimes you know you will probably come worst off by atacking the fort (getting a bad ratio and not even getting the fort), but it's worth the risk because you know that getting the fort would be a big bonus (eg delaying your opponent's 50 armies that were due to reach your cap in winter). Sometimes you know you probably would get the fort but it's too risky (eg you have 10 in England, 0 in Wales and he has 3 in a fort in Brittany and 3 to reinforce from Gascony - you could get the fort and be able to defend it for a turn, but you could also lose England which is 1 turn from developing so you don't risk it, even if it seems quite safe).