In my opinion, it varies depending upon each individual nativity:
-find the strongest (and therefore most positive) planet in your nativity
-take the test (always when Moon is waxing and Mercury is NOT retrograde)
a) when a benefic is transiting that planet (by conjunction or Parallel or semisextile or sextile or trine; never by opposition or square)
or
b) when that planet (your strongest) is transiting a benefic (Mercury, Venus, Jupiter) by the same aspects as mentioned above
or
c) when that planet is Cazimi the Sun (but never when that planet is within 5 degrees of the Sun-"combust"-either before or after the Sun)
or
d) when that planet is opposite the Sun (the ancients thought a planet is said to be very strong in expressing its own pure nature, when reflecting the Sun's rays, ie, when opposite to the Sun)
,,,if none of the above considerations can be applied, then choose when the waxing Moon is posited in the sign ruled by your strongest planet, or the sign in which your planet is exalted (but never when the Moon is passing through the Fiery Road, ie, the area between 15 Libra and 15 Scorpio)
NEVER TAKE YOUR EXAM DURING THE HOURS WHEN THE MOON IS VOID OF COURSE OR DURING ANY DAY UP TO THE PERFECTION OF A MOON/MERCURY OPPOSITION!