Are there any ways to figure how much Alimony a woman is entitled to in a Divorce?

I was going to file for Divorce from my husband of 29 years as we have been separated for over a year and he has no desire to get back together. He makes around $1600. month and I am on Disability and SSI and trying to raise my granddaughter, and he was giving me some money every month, but when I mentioned the divorce, he said no more money. I told him that I wanted to file for the divorce by myself and not get any attorneys involved since I thought we could agree on everything, but now he says that unless I put it in writing that I won't touch his Pension,his retirement and his Soc. Security.....that he won't give me any more money. I also have to agree to no alimony. I told him that if I get a lawyer involved, my alimony for 29 years will be alot more than what he was giving me now, plus, by law, I would be entitled to his pension and all that other stuff. I just want to know if there is some formula to maybe give me a general idea as to what I would be entitled to based on his income.