Longterm option agreements

[Neal Collins]

We've been working on a deal that involves the acquisition of multiple properties that we are getting option agreements on. The seller does not want to sell any of the properties now to avoid tax damages, which is why we have gone about the option route. Does anyone have experience with longterm option windows? We won't be able to execute the option until the estate takes over, but we need to come to an agreement that reflects market changes in case that isn't for a long time. 

We have toyed with CPI but that wouldn't correct for a downward shift in the market, and are now tentatively thinking the percentage change from the local RMLS reports. It'd be great to hear from anyone that has experience in this area.