The calculator
might be good (I did not try it), but from my experience a good rule of thumb is that your startup stock options are worth nothing.
The most likely positive exit is that your company (assuming you are actually at a startup and not something like Dropbox pretending to be a startup) is an outright purchase. In that case, VCs will take a preferred multiple of their investment first, should there happen to be anything left it will almost certainly go to direct compensation to retain the principles during and immediately following the buyout.
If there is anything left after that (almost certainly will not be) then your options might (but probably are not) worth exercising.
The notion that your options at an actual startup are worth anything related to "Post-money valuation at last financing round" is sadly laughable.
