I have looked at the standings and you are right they are very confusing. But I think the calculation behind the effective changes are correct. To Give an example:
In this pic you can see that my Caldari Standing is +6,86 and the standing for Gallente is just +2.22.
When I do a mission for Caldari (+5%), the half of the standing would be the negative to Gallente (-2,5%).
Now these percentage change must be calculatet on base of the rest to +10 (for Caldari) or to the rest of -10 (for Gallente).
This means I need 3,14 Standing points to reach +10 for Caldari. (3,14 + 6,86 = +10)
or I would need -12,22 standing points to reach -10 for Gallente. (2,22 - 12,22 = -10)
• 5% of 3,14 would be 0,157 so I would get +0,157 points for a Storyline with +5% for Caldari
• 2,5% of 12,22 would be 0,3055 and I would get -0,3055 points for the same storyline to the standing for Gallente.
I hope that this view is correct and that I am understandable. I really hope that the numbers are correct.