There is the issue that 320x160 is no longer even close to a safe assumption for what portion of the level a user may have visible initially, unless that's the level's size exactly.
Instead, if we're wanting something better than the crossed box that marks the center, I think it's best to display a region (or perhaps two or three regions, even) that would represent a fairly common case.
From what I can gather, the most common screen sizes among the community appear to be 1920x1080, 1366x768 and 1024x768. These would give default zooms of 5x, 3x and 3x respectively, which (once the screen area used for the skill panel is subtracted - and I'm going to again assume default settings, ie: not the compact panel, here) give display areas of 384x184, 455x229 and 341x229. Perhaps if we display the screen start position with rectangles displaying these three sizes? We could perhaps take out the one for 1366x768 - I'm not sure here. On one hand, it is a VERY common display resolution, but on the other hand, it's just shy of defaulting to one zoom level higher, which means it gives somewhat unusual results in terms of how much level area is visible at the default zoom.