I am using BuildTak on top of a Mirror. I use 230c for PETG & bed temp from 40 to 60. For PLA I am using 210c. I sometimes struggle with getting the part off the buildtak, especially really flat parts. The best tool I have used for getting the part off of the buildTak is a utility knife blade by itself. I am very careful not to cut myself and push with both thumbs the blade under one corner of the part to get it started. Once I am able to get it just a little under the part, I use a putty knife to slide under it. I add round disc ears to the slic3r on the corners for long parts to make sure it stays put. I was adding these ears to the original designs, but I was loosing the perimeter around the corners when I cut the ears away. I use a feeler gauge of .2mm to get the gap between the hotend & the bed. The 1st part I did that really stuck to the bed, I was not sure I was going to be able to get it off.