If you can not determine how much of your Box 5 is for health insurance, you can call OPM to ask them. You can contact OPM as follows: To contact the Office of Personnel Management (OPM), you can call their Retirement Services at 1-888-767-6738 (Monday-Friday, 7:40 AM to 5:00 PM ET), email them at retire@opm.gov, or visit their website by clicking here.
If you can find the annuity statement that OPM sent you for 2024, it lists the "Monthly Health Benefits" that are deducted from your pension. Multiply that times 12, and that is the amount of Box 5 that was for health insurance.