bed leveling and ambient temperature

hi, i was printing good until yesterday , i dont know what happens, i level my bed like always, i checked the axis some days ago and they where squared, my dude is what is the ambient temperature needed? can i print pla in a room temperature of 12-19 celcius ? or is too cold? my z endstop broke, so what i do is after level the bed i reset the printer so start at 0 in z axis, i always do the same, it broke even before i finish the printer and i dint buy a new one becouse that method works, but heres my other dude, the paper i use to level the bed has a thickness of 0.1 mm, when i tell the slic3r that i want a first layer height of 0.35 mm (the standar value) actualy is doing 0.45 scine the 0.1 mm of the paper?, or it do the math the arduino (using marlin)? after seeing the code i think is making layer of 0.45 becouse move the z axis to 0.35
bla bla bla
G1 Z0.350 F7800.000
and after this code is doing movents with the extrusion

Yes, 12-19C is fine for printing PLA.

Yes, the slicer assumes you start with the nozzle at zero and then it moves up to whatever first layer thickness you set. If you set the nozzle at 0.1 mm using the paper then tell the slicer to print a 0.35 mm first layer, it will print at 0.1+0.35=0.45 mm. However, paper is "soft" and your printer is probably a little "soft" (slop in the bearings, flexible guide rails, etc.) so when you set the nozzle to grab the paper you're probably pretty close to 0 when you remove the paper. Of course, you're doing this when the nozzle is heated, right? If you do it when the nozzle is cold, there will be a bit of hard plastic hanging from the nozzle that will cause an error in setting the z=0 point.

Ths is the only "dude" I know: [www.youtube.com]

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

i bought a filler gauge and level my bed with the 0.1 mm gauge, then i move the z axis 0.1 mm to "touch" the nozzle, to be totaly 0, of course i do this with the nozzle out of the heatbed and start to print, and the result is freaking amazing , perfects first layers always, my extrusion problems go away, i also tried with a paper and the result is very good too, i stoped a print in the middle and i could see that the first layer is like a glass perfect bond totally flat, that solve all my problems

just to add to what DD said:-
The 0.45 is assuming you have 0 in the z offset in your slicer. Normal practice is to set the Z offset to the paper thickness so you get the correct layer thickness. This is a negative value (- 0.1).
And it only affects the first layer since all the other layers will increase Z as per your layer height specified.