I live in Wisconsin as well and have to file for legal separation in the morning. My dh just got subpeona'd to appear in court and it looks like he might be the 'donor' after all. It's still a waiting game.
With me filng for legal separation to avoid paying as much c.s., my children will be considered first in the equation for c.s. So, x-ow will get about $200/month instead of $400+. They will not take into consideration our unborn child at this point for c.s. (our 3rd child is due in 12 weeks..I wonder if this is why x-ow brings this up after almost 6yrs of silence)
Our atty. told us that the c.s. amounts can be changed and/or reviewed every 34 months (can't be certain on exact #, but the amt. can be changed for a qualifying event...loss of job, birth, death, promotion, etc.).
So, even if you do file for legal separation (our atty. doesn't want any part of "saying" that it's to avoid paying as much c.s.) and get c.s. at this time, when the twins are 18, I imagine that the x-ow's dollar amt. would change and become more.
The other thing that my atty said was that we can disagree on the amt. paid and file financial papers to show that x amt. would be a financial hardship on us and let the judge decide. (instead of doing the legal separation thing) The 17% is just a guideline and we would have to prove that we couldn't pay the $400+/month. She said that the intent is not to make us broke and impovershed, but to keep her and child off of state aid, etc. (I bet to differ. I've read too many stories here!)
Oh, also, one of my friends was able to put her kids on state health care by saying that she was separated. She was separated, but not legally, she only had to give separate addresses.
Good luck!