When a property appreciates, it will appreciate on the value not on the original investment. Therefore, if you ‘leverage' the property with "OPM". You can make an investment even more valuable by having the property rented out. The monthly mortgage payment is being paid by the renter and someone else is paying for your equity growth in the property.
When you bought that new car you knew as soon as you drove it off the lot it would be worth 20% less than you paid for it. Twenty percent is a lot and more than most folks should be willing to risk when investing. Forget “the long haul” as you don't want to take the 40% losses that many investors did in 2000.
Usually a good rule of thumb is 10%. When you drive that stock off the exchange floor your risk should be limited. You decide how much you are willing to lose if it goes down instead of up and as it goes up carry that risk percentage along to lock in your profit.
Instead of getting a refundable deposit (like you would get from a renter), you get non-refundable option money. I suggest charging about the same as you would charge for the deposit... typically, 1 month's rent.
Check out what is customary in your area. Some areas are getting as much as 5% down on a lease option!