What can you say just looking at this picture?
->There are 2 obvious observations that can be drawn from your image.
First, the bed does not seem to be levelled correctly, the right side (especially the front right corner) is closer to the nozzle than the left and left-back side.
Is it possible to tell from this calibration print which corners are
not leveled?
-> Yes, the front right corner is higher. The filament is much thinner there, it is more transparent than the rest of the lines.
Second, you have glass mounted onto an adhesion sheet similar to BuildTak. Note that such adhesion sheets are rough (to create more surface area for the filament to grip on), placing a sheet of glass on top of such a sheet will create small air pockets (less contact surface area to the glass plate as that is perfectly flat by its production process) and thus an unsatisfactory heat transfer process. Glass directly onto the Aluminium will have a higher temperature than with an insulator (BuildTak sheet) in between it, it is far more difficult to determine the bed temperature that way. For printing on glass, you could also raise the temperature compared to direct printing onto the metal bed.
Should I increase or lower the temp?
-> When using glass on top of the heated bed, you should raise the temperature a few degrees.
You should remove the BuildTak sheet and put the glass directly onto the Aluminium heat bed or remove the glass plate and print on the BuildTak sheet directly, level the bed better or consider installing an auto-level (touch) sensor if your printer electronics board supports that.
What are some tips for printing on glass?
Furthermore, use some PVA based spray (e.g. certain hairspray brands or specific print adhesion spray) or Polyvinylpyrrolidone based glues (certain glue sticks, white wood glue, etc.) to create a sticky surface for the filament to adhere to. I have great experience with PVA based sprays like 3DLAC, but there are great alternatives found in common household hairspray cans as well, as long as it contains PVA.
Or is this just a general lack of stickiness issue (e.g. i need glue)?
-> Yes, Polyvinylpyrrolidone/PVA based glue/spray will result in better adhesion.
A general remark is that you could listen at what others have tried, but you need to find out yourself what works best for you. In order to get the print to stick to the glass, you should use every trick there is to get good adhesion. If PVA based sprays or Polyvinylpyrrolidone based glues work for you, use it, if not, print on bare glass, use tape, BuilTak, elevate bed temperature, overextrude first layer, use brims/mouse ears, etc..
