Divorce Support Forums: need help calculating approx. support amount.

Quick background:
1st ex is paying child support for 1 child, age 13, based upon calculations from 11 years ago when he was in military and had it lowered. I have never asked for a modification as I held that in my pocket so he would allow me to move out of state with our son. I have lived in Fl now for over 3 years and I pay 100% transportation. He takes no abatements when he has him in the summer, and that is the only parenting time that he now takes (his suggestion last year to save me $, and so he wouldn't have to review his CS amount). He provides $50 toward 1 sport per year, and specific school clothes & supplies which amount to probably $100 (if you saw my cheap shoes thread, you'll see what I mean)
He now is asking that I look at what I will be seeking in 18 months as that was when we agreed to next determine any changes. Here is the e-mail from him or his wife:

***This is going to sound like a really weird question this early in advance, but we need to ask it anyway. What are you thinking for the next review?
The reason I ask is that we really need some home improvements but in order to make a commitment like that we need to know what to expect our total bills are or will be. As you know we are on a very tight budget, so we always need to make sure of possible changes before hand.
What we were thinking was keeping everything that is now plus doing $50 towards transportation(plus the gas to pick him up in either Detroit or Lansing or Chicago seeing how those are where the cheaper tickets are). Because he will be attending high school at this point, giving $80 towards his class ring(that is how much we paid on M's), plus $50 towards driver's training(which we are making everyone here pay for their own or wait until graduation). We are also willing to send some toiletry items for the year with him when he returns for the summer-like toothbrushes, toothpaste, deodorant, and vitamins. That would be 12 toothbrushes, 12 tubes of toothpaste, 12 containers of deodorant, and 12 months worth of multi-vitamins. Does this work for you?***

I answered back that it's too early for me to know what costs are going to be and that as N gets older it does cost more than the $12 per month extra just to feed him...so no, this isn't going to work for me. Along with information on what it costs to insure a teenage driver, although I don't expect them to cover those costs it is relevant to my own budget.

I don't think I want to roll the dice by doing a review through the state, as he has since adopted his 5 step-kids and I really have no idea how much money he makes but assume it's only around $15 per hour. I've been told by my own attorney that they probably wouldn't raise his support just because of his obligations and also included right now in his support is daycare that I obviously don't need anymore but without that his support would be less than $45 per week.
Help...I need to start getting an idea on what I'm willing to accept. Any suggestions would be appreciated.

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