I don't know the best setting for my water heater

[worriedme]

My Bradford-White fairly new water heater's dial had to be turned to "vacation" while a plumber worked inside on the shower. In turning it back on, I see one line, then a box with an x in it, then the third line. I don't have a manual for it. I have it turned to the first line, but the hot water runs out before too long, so I need to adjust it. I'm just worried about the box with an x, and I can't remember what the original installer told me.
Can someone tell me what the box with an x in it means?

 

[Geofd]

What type of work did the plumber do??????
Just turn the knob a little at a time,it will probably take hours after each adjustment to show results