This is not hard to determine, We can simply use the voter calculus: The chances of being ranked in the top 25 are:
(W/(1+L) + R + H + (25 - PR/3))/ G = BRPI (Being Ranked Probability index)
where W= wins, L= losses, R = wins over ranked teams, H = wins over highly ranked teams, PR = previous ranking (PR of unranked teams is 0), and G = games played
If the BRPI is >= 1 you will be ranked.
This equation is guaranteed to be 100% accurate 75% of the time.
Please note that the equation used to be (W/(1+L) + R + H + SB+ (25 - PR/3))/ G = BRPI where SB was 1 for any SEC team, and 0 for anyone else. The SB adjustment has been removed for this season.
What could be easier. We see that Nebraska's numbers are:
current: ((2/(1+0)) + 0 + 0 + (25-26)/3)/2 = 0.83
Win over the Ducks : (3/(1+0) +1 + 0 +(25-26)/3)/3 = 1.2
Last year's final rankings: (6/(1+7) +1 + 1 + 0 + (25-26)/3)/13 = 0.18 (note: last year's equation had the SB cofactor that is gone from this year's equation, though there is a move by ESPN to have it reinstated)