Hmm, I've never failed to capture a ship, and I've taken both large transports and battleships with a single level 2 boarding party. I do tend to climb right up to level 5 boarding parties very fast (since its such and affordable tech once you get past the military science level 2), but in all my time playing, with the exception of testing security stations against myself, I have never once failed to capture a ship without security stations no matter how few or how weak my boarding parties. I guess it's just that I upgraded my boarding party quality faster than I paid attention to or something.
As for the 1.5:1 ratio, yeah it's an approximation, but not a bad one. If you know the enemy has 1 level 1 security, either bring a level 2 boarding party, or 2 level 1's and you should be fine. If you only use 1 level 1, you'll lose. If he's got 3 SS 3s, then you want about 5 or 6 boarding party 3s to take him. Or, you can do it my preferred way and just board the ship right back from him...
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