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I see conflicting information from various websites on backspace measurement.

using the following data: 19x8 w/ +1 offset, this website shows 115mm (4.5"" backspace) measured from hub mounting surface to inside bead flange of the rim.

https://www.wheel-size.com/calc/?wheel1=285-70-17X7.5ET45&wheel2=325-65-18X9ET1&fcl=12mm&wcl=100mm&scl=12mm&sr=12mm

This website shows 5.0" from hub mounting surface to back of rim.

https://www.rimsntires.com/specspro.jsp

Do most rims/wheels factor 1/2" (12mm) lip to make up the difference?
 

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Yes, there's a 1" tolerance considered for the lips. So, a 9" wheel would be calculated on 10", which means your centerline would be 5". So a +1mm offset would be just a hair above 5" backspacing.
 
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