Unless your intern is insisting on definite specifics (which would be a bad sign) I'd suggest you keep it vague rather than trying to anticipate all sorts of scenarios, and still probably end up with something unexpected!
Furthermore, the whole concept of allowing an employee to buy shares, rather than being given shares or share options, is slightly out in left field. Just who exactly is doing who the favor here!? Not only is there almost surely considerable risk, but the value of the shares will probably be a very vague sort of concept, because there won't be a free open market for trading in your shares (I presume that you have a startup that is still full of risk and some distance away from an IPO or any major market valuation validation).
I also don't know if 'qualified investor' considerations/restrictions apply to employees or not, but it is something hopefully some other response will advise you on.
You have to be careful here - if you are trying to equate a specific value to this dubious privilege, then there's a small risk/liability you're opening yourself up to if the value is not realized. I suggest you view this as a nebulous and impossible to quantify additional benefit, rather than part of the core package you're offering.
The reality is that sure, the person could buy shares at a theoretical high value/low price, but could they then immediately sell them on and make a profit? If the answer to that is no, and I'm guessing that is indeed the answer, just how substantial a benefit are you offering?
So I'd suggest going vague on things and say something like
Part of your total employment package will be an opportunity to purchase shares in the company at below market value. We can not today predict what the market value will be in July 2017, and indeed, because we are not and will not be a public traded company, the market value is hard to establish in any case, and any such purchases of course come with risk and no guarantee of appreciation.
It is our intention to encourage employees to become shareholders, and to issue some shares to employees for lesser amounts than we are generally selling them for to other outside investors. We'll talk the specifics of how many shares, at what value, and the basis for setting that price, subsequently.
TL;DR - I think the whole concept is a bad idea.