Considering Marco left Tumblr a year before [1] they raised their last $85M round at $800M valuation [2] do you you think it realistic/possible that he had 1/8th the company after that deal?
My rule of thumb is co-founders have 2/3, employees have 1/3, and investors cause a 75% dilution per round. Thus since their are 4 rounds, .75^4 * 2/3 => 21% between the founders.
However, in this case it appears that Marco wasn't a co-founder, but rather a first employee with an equity grant. If it were for 5%, it is probably worth ~1-2% now, and so I'd reckon he will see $15M. That is a lot of money, but not enough to change the startup landscape.
[1] http://techcrunch.com/2010/09/21/marco-arment-instapaper/
[2] http://techcrunch.com/2011/09/26/tumblr-raises-85-million-ro...