PIC 18F26K83 ADC clock frequency
You shouldn't need to take this into consideration. Fosc is the oscillator frequency (system clock). FCY is the instruction clock which is Fosc/4. The selections for CS in ADCLK refer to Fosc and not FCY. see table TABLE 37-1 in datasheet.
division of 16 should give you conversion clock of 2us (if Fosc=8MHz). You'll notice increments of two per 1 bit and starting at 2. 


    div 2 =  000000
    div 4 =  000001
    div 6 =  000010
    div 8 =  000011
    div 10 = 000100
    div 12 = 000101
    div 14 = 000110
    div 16 = 000111


i.e. 

ADCLK.CS = (div / 2) - 1