I'm new here also, but I would think the trucked in water was like my city water. If that's the case, as mine was when I refilled it with city water a couple years back, I was told it was the same as taking a bath and hop right in. If the trucked in water is NOT filtered and treated water (such as city water) I would test it right away and base my decision on how quickly to hop in on that. On a side note - I did full tests on my city water so I had a base line to know what was publicly safe and to know what I was putting in (chemical wise) in the future. Experts with more experience will have way more valuable input than me.
If it's not city water, but clean, I would think you could get a guesstimate at half full and calculate what would need to be added at full (based on volume) and begin adding it then.