Please be aware that extension is for filing - not for paying taxes.
If you owe additional tax - it would be better to pay estimated amount at the time you fill an extension to avoid additional penalties and interest.
The tax liability is calculated based on the total taxable income - there is no pre-set percentage...
If you retired on disability, you must include in income any disability pension you receive under a plan that is paid for by your employer.
If you receive a disability pension based on years of service, you generally must include it in your income. However, if the pension qualifies for the exclusion for a service-connected disability, do not include in income the part of your pension that you would have received if the pension had been based on a percentage of disability. You must include the rest of your pension in your income.
Amounts you receive as workers' compensation for an occupational sickness or injury are fully exempt from tax if they are paid under a workers' compensation act or a statute in the nature of a workers' compensation act. If your disability pension is paid under a statute that provides benefits only to employees with service-connected disabilities, part of it may be workers' compensation. That part is exempt from tax.
Disability Social Security payments may be partly taxable. If your other taxable income plus half of Social Security payments is below $25,000 (for singles) or $32,000 (for married couples) - none of Social Security payments are taxable.
Let me know if you need any help with estimations - but in this case we would need additional information.