First off it must be 100% cotton ticking second I have found it in 3 thicknesses third as mentioned above you must washer out the sizing I use hot water and tide to get it out and Wash twice then air dry.
With calipers I find the sizes to run about 15, 18 and 22 thousands with a hard squeeze on the calipers.
There are some substitutes out there that some sales people will try to con you into buying they did not work for me.