I ran several simulations over the weekend. Here's the bottom line: the "divide by 6" rule of thumb actually is slanted in favor of the Germans, and it's a mistake for him to make the attack. Russia puts 19I, 2F in Karelia. The Russian navy moves to the UK.

Germany brings in 11 I, 7A, 1B, and either 4 or 5 fighters. Let's examine each case. If he attacks with everything, he'll have, on average, 7 units, some mix of armor and planes at his discretion.

If he diverts one fighter to attack the British navy, he'll have 5 units left. It is right around 50% whether he'll actually sink a British transport or just the two Russian units. That's right, there's approximately a 50% chance that the UK will have two transports at its disposal to counterattack Karelia.

Now to counterattack cases: The German brought in everything, and took all planes as losses, as advocated. The German has 7 armor left, but the UK has 2 transports, and can bring in 4 I, 2F, 1B, and 1BB shot. Result: the UK clears Karelia (and probably retakes it in exchange for a fighter dying). Note that Germany has no planes or navy left to sink any allied navy.

The German sent in 4 fighters and attacked the allied Navy, and took all planes as losses. He has 5 armor left, and the British have either 1 or 2 transports and a battleship left. With 2 transports it's trivial for the Brits to retake Karelia, and even with 1 they'll take it (or at least clear it).

In either case, Germany will not get to build in Karelia, and the Allies will be building an Atlantic invasion force. If Germany took planes as losses, the allies are ecstatic. If he didn't, then they re took Karelia fairly easily.

By the way, for those of you advocating the "attack Karelia one turn and withdraw" option, remember not to use the transport to bring in extra infantry, because then it's an amphibious invasion and it's a battle to the death. Stupid rule, but it's there...