Sticky notes work good for nozzle spacing. Adjust for just a very slight tug on the paper. Make sure the nozzle doesn't have a booger hanging off the end of it.
I like to use 170-180º when setting it so the filament doesn't ooze out too much.

Use a straight-edge and look for hills or valleys across the bed. Use a light behind the straight-edge ruler.
I use thin vellum or drafting paper as a shim under the flex mats to fill a valley in the middle of the heater.
Glass should be dead flat.