to avoid cross eyed viewing a smartphone infront of my face i tested reading glass.

i used cheap 10 euro reading glass with head mounted smartphone(sony z2)/tablet(sony z3 compact)
This works very nice, better than cheap fressnel lenses.
Reading glass gives very brilliant image and is very comfortable to wear.

+2,5 dioptrien is usable, but +3,0 or more dioptrien would be great, because
the distance of the smartphone/tablet is still more far away then the x4 lenses of google cardboards.
You can buy +3,0 dioptrien easily, but more dioptrien..

17,00 euro
http://www.sehhelfer.de/index.php?cl=alist&searchparam=&cnid=42f48c815c0948b12.20857754&attrib[Dioptrien][]=4,0

i took a second identical reading glass and removed parts of the frame.. so i could use it
as second glass directly on the first glass.. this works.. but there is a 2-3 mm air between the glasses… the impression is not as good with single one, so further test could be
to buy reading glass lesnes without frame and use them directly on first glass.

so the best solution would be a single glass with high + dioptrien..

http://komerci.de/informationsservice/dioptrien.php dioptrien vs. magnification factor
google cardboard has 4x magnification factor.. this means a dioptrien number of +16 !!!
our reading glasses with 3 dioptrien have only magnification factor of 1.75 !!!