Generally, monthly payments (estimating a 100% LTV, which is rare now) run about 0.8% of the sales price, but can be more or less, depending on the tax and insurance rates of the exact community.
For 0.8% to work out to 30% maximum of your gross monthly income, you would divide and find 37.5 times your monthly income (or 3-1/8 times your gross annual income). Bear in mind that this is a rule of thumb, not an industry regulation.
The 0.8% comes from $5.37 per $1,000 of mortgage amount at 5% for full PITI [0.537%], plus 2.5% per year for real estate taxes (some places around Frisco have 3.1%, some 2.3%) [0.208%], plus for insurance a range of 0.5% to 1% per year for full coverage [0.042 to 0.084%], for a total of 0.787 to 0.879%, depending on the tax districts and selected insurance coverage. The interest rate of 5% is typical but can be higher or lower. Some big name mortgage brokers claim it is much higher -- just do a little shopping.
The 30% of gross income is also subject to what your total debt payments are per month. Some guidelines allow up to 45% of income for all debt payments and 33% of that for housing, but some guidelines for other loan types allow up to 41% total for housing or all debts. There is no single target. So, 30% may be a little conservative but may be too high if you have a lot of other debts to pay each month. Other payments include credit cards, car loans, child support, installment loans, etc. You don't have to include current mortgage unless you're keeping the property.
Of course if you put more money down and get a loan for only 80% of the sales price, your payments will change. The PI part will be 20% lower but the TI part won't budge, because they're based on price, not loan.
Aren't you glad you asked? At the moment use 3x, unless you know you can't afford it due to other payments.