How can I determine the surcharge for Medicare Part B Late Enrollment?

My spouse became eligible for Medicare Part B in February 2005 due to disability. She delayed enrollment due to creditable coverage thru my employer. Fast forward to July 2010 when we lost coverage due to job termination. Health coverage was thru Cobra from July 2010 thru August 2011. I began working again and creditable coverage resumed thru my employer until I retired in 2016. My spouse attempted to enroll in Medicare Part B in July 2016 thru a SEP, but was denied due to the earlier Cobra coverage. She enrolled in general enrollment, but has a late enrollment surcharge. The question is what should the actual surcharge be given there was only a single 12 month period under Cobra or does the "clock continue ticking" even though creditable coverage was restored when I became re-employed? My spouse i not yet 65 and I understand that the surcharge goes away upon turning 65.