As I understand it, you take 90% of your last rep record's predicted max. Predict by weight * (reps * 0.333) i.e. 100 * (5 * 0.333) = 166.5.
N.B your predicting max will be slightly different depending what week you take it from. Wendler suggests taking the higher predicting 1RM (e.g. if you did 50 x 12 on a light week, say the 5's week, this will give a higher max than the 100 x5). These numbers are arbitrary, by the way. Nor does it matter in these examples about pounds or kilos. The formula works for both