NSS Breeding - Even, Odd Pairings?

I've been here long enough that I should really know this, but I am newer to the world of breeding for low NSS. I've seen some guides on trying to breed pairs that have mismatched even-odd NSS stats, because that helps pull them lower. My question is, is it better to pair horses with even-odd NSS that are further apart (for example, 21 agility and 10 agility), or even-odd NSS that are closer (for example, 14 agility and 13 agility)? Which pairing would have a better chance of lowering the agility NSS, in this example? 

technically both are lowering the nss at the same rate, the first example would result in 15.5 which rounds down to 15 nss while the 2nd example is 13.5 rounded down to 13 nss. pairing evens and odds just shaves down an extra 0.5 each time, as opposed to even/even or odd/odd pairs that don't get rounded down lower.

that makes SO much sense, thank you!!