Ron

Definitely use your chrony result that you measured.
The takeaway from this is that your measured velocity is lower so your pressure should be lower. That's the way I see it.
QL is only a program so it's only as good as the data inputted. If, for example, your batch of powder had a different burn rate to the data in QL or it was affected by your different climate, it will give a different result. I remember stumbling across a discussion on here some time back about how IMR powders were climate sensitive and H powders were more stable. I think Daryl S was involved in this. Maybe he could chime in. Anyhow, not to drag the thread off topic.