You almost had it right Fratello.
And the timing will be more accurate if you don't do the math on each loop.
Code:
vmax = 0
for cnt=1 to 70       			
    adcin 3, v_cal
    vmax=vmax max v_cal
    pause 100
next cnt
v_cal=vmax * 5000            ' I use VDD as Vref
v_cal=div32 1023