Bed height is important Using paper and card is fine bu ta bit it and miss. since we use our machine almost eevry day and often all day!! we found we needed to be a bit more precise so i bought 3 pieces of thin steel "shim stock" from ebay. they were specified in thousandths' of an inch 0.002" 0.003" 0.004" 0.003 is about the thickness of 80grm printer paper or 0.07mm thick I use this size most of the time but chech the nozzle ht at the point I expect the print to be printed as well as at the corners and adjust the corners if it not within the tolerences of the 3 shims. Another advantage of using the steel shims is that if you turn up the corner so you can hold the you can level your bed while it is hot. The ender 3 bed seems to expand a little unevenly.Also try glue stick it is easiest to apply when the bed is hot