I've started my floor leveling over a joined living room and dining room (divided by a double door entry). I think it is going OK after my first pour of two bags of Levelquick. Though, I do need more SLC as I apparently did not put enough down the first time.

Here is my question: From a measuring perspective, I am getting slightly different measurements as I position my string at different points around the rooms. I can measure an area one way and note more than the usual 1/8" difference, then when I move my string to, say, 90 degrees off (or 180 off), I may see much less than the noted 1/8". So, I'm saying I am getting different measurements of the same area as I move the measuring device around the room. Is this normal? Do I simply take multiple measurements and make my best guess?

While a string is good for brick layers and making a straight line, it isn't the best tool for finding high and low spots. The pros don't use that approach so that should tell you something. Go to the lumber yard and find the straightest, driest 10 ft. 2x4 you can find. Move that around in different directions to locate high and low areas.

I do not use that string method either. A few years back I started using a 1"dia. long and straight black "gas line" pipe. I can roll it around and easily locate problems. Low spots you can obviously see underneath, and it will teeter-totter over the high spots.

If you try that do not use a plastic pipe, it has to real rigid.