Basically yeah, for each one take (load/tap) to give you the fraction of impedance and combine them like parallel resistors so if you worked out:
load fraction = 100 * 1 / ( (tap1/load1) + (tap2/load2) + (tap3/load3) ) that would probably be right.
So if you had a 16 ohm on the 4 and a 16 on the 8 for example load = 100 * 1 / ( 4/16 + 8/16) = 100 * 1 / (3/4) = 100 * 4 / 3 = 133% impedance
something like that. Of course your power would be split funny too, that's the other issue, although this impedance isn't TOO far off one of your speakers would be getting twice the power of the other one, that might be why people suggest that this shouldn't be done.