There is no perfect answer to this question, and you will always have a tradeoff between likelihood of executing the order and the price you wish to achieve.
The only way to be certain to execute the order is to sell at the bid (or more accurately at the highest bid that immediately accommodates the full volume you are trying to sell), and the market's price can move away from you as easily as towards you.
Depending on your broker, there may be liquidity they can access that isn't immediately obvious to you (so-called "dark pools"), so it may be worth trying a high initial bid, just under the ask... Your broker may also have algorithms that purport to execute inside the spread only: IB has tried a couple ways of doing this, but my limited experience with these algorithms hasn't been stellar (though I haven't tried their newest version).
My approach for wide spreads if I need to sell but am in no particular rush is to set my limit just under the current ask, wait 5 minutes and look. Often a new ask will have appeared just under me. Rinse and repeat until you converge on the minimum price the other potential seller is willing to settle for... then wait. Towards the end of the day, start lowering towards the bid if you really need to sell. This approach doesn't guarantee anything except that if you execute, it will be at a "reasonable" price (in the sense that you will have aided price discovery and added liquidity to the market... anything inside the spread is objectively reasonable in that sense). Setting your limit outside the spread, especially when the spread is wide, almost guarantees that you won't execute in my experience. The exception is when your security usually trades only in very large chunks, but this happens usually only with some weird ETNs, and even then you're gambling that the next large liquidity taking order will be the opposite direction you are trying for.
The way that vanilla long-only equity fund managers usually do this is to request "VWAP" from their brokers. VWAP stands for Volume Weighted Average Price, and is essentially the price you'd get over a period of time if you took a proportion of each trade. Their brokers will then do their best efforts achieve that price, essentially by trying to be part of every trade during the time period, in the correct proportion. They have to forecast things like tick-by-tick volatility and depth. It's not something that you or I could do at home. They usually do better than we as individual investors could do, but the difference is not as big as you'd think, and in less liquid stocks sometimes the best achievable is over a percent worse than VWAP. I don't think they're ever trading stocks quite this illiquid, though...
It's worth keeping in mind that this whole game is just that... price discovery is exactly what the HFT people work so hard on, and it's how they make their money. Their raison d'etre is narrowing spreads, and if they have not succeeded in narrowing the spread in the security you are trying to trade, it's almost certainly not worth your effort to try to beat them here.