Do not enter the "health benefit" amount in Box 15 of the TurboTax Form 1099-R interview.
I'm not sure why your former employer is using that box to report your "health benefit" (actually your insurance payments), but that is not the correct place to report this information.
If you are using a desktop version of TurboTax (CD or download), switch to Forms View, select your OPM 1099-R and enter the amount of "health benefits" on the "Insurance" line - see the attached screenshot example ($888).
If you are using an online version of TurboTax, enter the Box 15 amount as a deductible medical expense (insurance premium). Enter "medical expense" in the Search Box and select "jump to medical expense" option.