Symbolic evaluation (float) within function assignment?

I'm trying to use a floating point evaluation of the LambertW function, within the assignment of a numerically evaluated function, but once the argument to the LambertW function becomes dynamic (i.e. the numerical wrapper function's arguments passed into the symbolic LambertW function), the LambertW function turns red and shows the error "This variable is undefined." I've been searching the archives of this forum and I see a few posts with similar questions, specificallly pertaining to the polylog symbolic function, but frankly I don't understand the discrepancy yet. If anybody has time to check out the attached worksheet and give me some pointers, I'd appreciate the help. Thanks!

Update...

Been working on this... I still don't really understand the disconnect between the symbolic and numerical solver, but after searching this forum more, I found a few worksheets and posts that provided some insight into numerical evaluation of the LambertW function. Specifically, this post:

http://communities.ptc.com/message/58209#58209

Using examples in the worksheets attached to the above post, and with a little tweaking of the initial value for the root function, I came up with a solution to my function that appears to be working well (attached).

Thanks to everyone who's posted about this in the past - I learned a lot from browsing old posts!