David,
Around SW TN we get a lot of this kind of thing. 175 acres with wild timber, most of which is merchantable. What's the value? Well, we find that row crop land sells for about $1,000 to $1,100/acre. We find that land with standing, merchantable, wild timber sells for about the same. Cut over land sells for anywhere between $250 and $800/acre, depending on tract size, topography, geometry, access and appeal. So timbered land is worth $1,000 to $1,100/acre around here -- if it's got merchantable timber. Might go for a little more with exceptional timber; there've been a few timber sales at $2,000/acre.
The HBU is "residential subdivision" only if a market exists for a residential subdivision at present in that market. Sounds like your tax guys are pulling a stunt with that HBU classification. Just because you {can} subdivide, doesn't mean you {should.}
DCF on a ten-year holding period? Upon what data would you base lumber values ten years from now? From what sources would you derive a yield cap rate? How would you project those rates that far into the future?. Hell, even the reversion is a squinting guesstimate that far out. Man, I hope you're getting a better fee than those timber cruisers for this if you're going to do it by DCF. Supporting the assumptions will be a suffering b*tch.
I really think you're "over-thinking" this. I think the correct value is indicated by current vacant land sales having the same HBU and similar timber. It may be difficult with no public records reflecting timber, but that's why you drive the comps (or fly'em, for large tracts) and interview the parties. PITA to be sure, but it should be a well-compensated pain.