For those who want to crunch numbers here is the Excel file I created for the comparison with all the scenarios and valuations:
I crunched some numbers to model different scenarios and different start-up valuations to compare whether the YC deal is any good and what the other options are. In general, thinking about the 7% of the Safe and whether it's worth it is the wrong mental model to start with. It’s like announcing winners and losers in a race immediately after the start and before anybody even runs half a distance. The thing is 7% Safe is just Step 1 in a priced round when YC’s shares convert — and there are 3 steps that depend on the priced round valuation and share a new VC gets. Anyway, I modeled 3 scenarios with 7 different start-up valuations of the priced round for each to compare founders' shares as a result.
YC also has a 4% participation right https://www.ycombinator.com/deal/
Include that and founder ownership drops by 4 percentage points in scenario A.
Also AFAIK the option pool is usually 10% post-money, so the founder would get a few percent less than calculated in the examples (10% pre-money).