I am confused as to what your asking. When you place a home up for sale the person buying the home will satisfy your existing mortgage and you will get whats left over (what your calling equity).
You may take this left over money and use as a down payment on a new house. I believe your thinking you want to spend about the same on a new home as your current homes value. So your down payment will roughly be 40% - 50% of the purchase price (60k down)
The way to figure out how much of a monthly payment on a mortgage you qualify for is as follows. This formula is used no matter what your credit situation is.
1. Take your total monthly income (before taxes) and write it down.
2. Then write down all of your monthly credit obligations that get reported to the credit bureaus. Examples are child support, credit card min payment, student loans, car payment. You may or may not have any of these. Your light bill, tv, phone, and other utilities do not count.
3. Add up your monthly obligations.
4. Take your monthly income and multiply it by 43% then write down the result on the paper.
5. Lastly, subtract the total monthly obligations from the number you got after you multiplied by 43%.
This is the max a lender will allow you spend per month on a mortgage. All lenders calculate this the same way it is called DTI or debt to income ratio. Some lenders have some exceptions. If you are a really strong applicant with awesome credit they may use up to 46% or 47%.
Once a lender has figured out what interest rate you qualify for they will look at your requested loan amount and see if the payment will fit into your DTI.
Farther more, having a healthy down payment is no replacement if your DTI is out of line.
Hope this helps