Yes, the capacity it has would be measured at the rate it will be used. Typically, easiest to just do that on the road, returning to close to home near the end of the discharge. Then ride around the block ten times if that's what it takes to be close to home when you hit 0% usable capacity.
Your capacity WILL vary by factors that affect peukerts. Rate of discharge, age of cell, and particularly temperature. Anything that changes the internal resistance of the cell, will cause a change in the peukerts.
But yes, compared to lead, the variation with lithium batteries is much less, but its still there. The closer to the end of the discharge you get, the more your IR increases, so your peukerts changes as you discharge every cycle. To maximize usable discharge, lower the rate once you get below 75% discharged.