How do I tell how much space I have between the plaster and lath?
Straighten the long leg of a large paperclip out, then bend the tip into a 1/8” “L”. Drill a hole through the plaster until it hits the wood lath. (You may feel the drill jump a little bit if there is space between the plaster and lath.)
Stick the bent end of the paperclip into the hole until it hits the lath. Then, using a marker, make a mark on the paperclip, where the plaster surface meets the paperclip. Next pull the paperclip out of the hole until the “L” catches on the back side of the plaster. Make another mark where the plaster surface meets the paperclip.
The distance between those two marks is how far the plaster is from the lath.