Very few companies have a revenue multiple of 13x. Looking at this chart https://blog.intercom.com/wp-content/uploads/2014/07/4-forwa... it looks like Dropbox can expect something in the 6x-10x range, valuing them at $4.5-7.5B.
That being the case, DropBox probably isn't profitable yet which would throw their P/E through the roof and make that a useless metric.
Box is already public and it has revenue around $300 million and a valuation of ~$1.9B. DropBox will probably get a better revenue multiplier since their customer acquisition cost is lower than and their overall network effects and future outlook is better than Box. I'd place DropBox at a ~$6B valuation. Hopefully their revenue grows enough between now and IPO to support something closer to $10 or $12B.