If Violin goes out at the high end of its range ($10), GSVC records a 66% gain on overall investment, that would be worth about $24MM to shareholders.
There are a couple of points here - first, as a later stage investor, GSVC is really playing the private/public valuation gap (if there is one). With FB, the private market had already whittled that gap to $0, or even negative. With lesser-known companies like Violin and Chegg I'd expect there to be a bigger opportunity.
On Twitter, one may be better off selling GSVC before the actual IPO.