URM14 requirement for precise measurement

Hi everyone, 

I want to measure a distance between URM14 sensor and rectangular wood piece size 20cm * 10cm. I want to know should this wood piece be larger than detected area depicted here ?
dfimg.dfrobot.com/nobody/wiki/41d5020e5b2ca8b2a5b231a8b07a91ad.pdf

what will happen if it is partly in the detection area? (for example when it has 40cm distance with sensor)
Precise measurement in our case is important.