Hmmmm.......... I've played the "Evacuation" mission several times (starting from Batavia), running at about 11-12 knots and I've burned a LOT more fuel than 30% by the time I've reached Denmark Strait, more like 50+%. To be as economical as possible, I hardly run submerged up to that point, only diving for about 3 hours whenever I get a warning about radar transmissions being picked up. The approach to Denmark Strait is where it becomes tricky, though. Since the SH4 IXD2 doesn't have a Schnorchel, I run at flank speed to cover as much ground as possible on the surface before I'm actually spotted, then as deep as possible at silent speed until battery or CO2 force me to surface, then flank again on the surface for as long as possible, or battery is recharged (whichever comes first). So far I've only managed to reach Trondheim once (only to be well and truly clobbered on the final stretch to Bergen
), but still had over 10% fuel remaining when I reached Trondheim.
By 1945 refuelling at sea would have been almost impossible, but U-181 could have reached Bergen without refuelling, otherwise she would not have tried to return to European waters.
Using 10% of the fuel to reach the Cape area might be realistic- what speed were you running at, what route? I think the IXD2's most economical speed may be around 8 knots, but unfortunately I can't calculate it without knowing how much the auxiliary load is.