In an application note for PIC24F16KA102, when calibrating the CTMU current source, it is recommended to set current and charge period to reach a 70% of full scale in ADC reading. Is there a reason for this? Would this provide the best conversion accuracy for a SAR adc?

http://ww1.microchip.com/downloads/en/DeviceDoc/39724b.pdf

Section 11.5.1

 

Like the note says, it's to put the reading well above above the noise floor of random fields or thermal noise affecting the device being measured.

http://www.daycounter.com/Calculators/Thermal-Noise-Calculator.phtml