As you say 2/3 of 16 rounded up would be 11 but I think I recall seeing a post somewhere where numbers like this were rounded up to the nearest even number but I might be getting confused with another rule set. If that were right it would become 12, can't for the life of me find it now though. That said, in Magna, again AFAIK, Tugs of 6 become 4. If we were playing we'd just use the number as 12 (given 11 is closer to 12 than 11 is to 8 ). I see the 2/3 as an approximate guide which needs to be tempered by playability. That said, in Magna, again AFAIK, Tugs of 6 (if that's the only option) become 4.