Tickless Idle losing time

Using an nRF52840 with SDK 15.3 FreeRTOS 10.0 (Nordic code) project.  I've got a vApplicationTickHook() defined that checks up ticks and fires a seconds counter at tickHZ rate from the up ticks:

    if(++UpTicks >= configTICK_RATE_HZ){
        UpTicks = 0;
        RTC_OnSecond();
    }

If I use tickless Idle, I see it loses about 3.4% of seconds over the course of 45 minutes.  If I don't use tockless Idle, I only see about 0.6% loss of seconds.

 I'm thinking that the thing is sometimes sitting more than a Tick in idle (which is reasonable I suppose in tickless idle).  What should I use to decide how many ticks I'm actually sitting idle with then?

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