My guess is that wherever you are, there is a similar page which you can direct your husband to for reference. Plug both of yourselves in there. If you are splitting 50/50 then you should add and divide by 2, and he should contribute the difference based upon the income disparity.
If he balks at this, it's a 5 minute trip down to your local enforcement office. I've been to mine, and it has shaken my wife up quite a bit.
M:40 W:40 2 teenagers ILYBNILWY: 09 January 2010 soon to be walking away my situation