# Logic of using Parent's PE when valuing RemainCo and SpinCo of a spinoff?

Reading an older book by Maurece Schiller, there is a section about ways to value companies planning / undergoing a spinoff. In it, he uses the parent's PE multiple to value the earnings of both the SpinCo and RemainCo. This is a bit confusing to me, since I would think that it would make more sense to use multiples from businesses whose fundamentals are more closely comparable to the separated businesses (the way I usually see relative valuation done) and the use of the old parent multiple seems a bit arbitrary. From the book...

For example, if the parent company stock is priced at \$60, based on 12 times earnings of \$5 a share, exclude the subsidiary’s contribution of [assumed in this example to be] \$1 a share, and then calculate the price of the stock. This would be 12 times \$4, or \$48 a share. The next step is to estimate the value of the subsidiary’s stock based on 12 times earnings. If we assume the subsidiary earnings of \$2.50 a share [the total undistributed earnings of the subsidiary in this example], then the estimated value would be \$30 a share. Adding the values of the parent and the subsidiary shares of \$48 and \$30 respectively, we have an indicated worth of the parent stock of \$78 a share as a divestiture situation.

Can someone explain the logic of using the parent's multiple rather than some other comparable business here (assuming such comps exist)?