The cost of health benefits for permanent employees is shared between the government and the employee. The government's share is approximately 2/3 of the cost and the employee's share approximately 1/3.
For permanent employees on a part-time work schedule, the government pro-rates the amount it will pay. The employee will be responsible for their share and the balance of the government's share.
Get more details at: https://www.opm.gov/healthcare-insurance/healthcare