Sale and Use Tax audit -NRE, R&D

I'm looking for any assistance related to sales and use taxation rules of purchased R&D activities, and what folks have run into on audit around the labor portion of R&D charges from your out of state vendors - we are in semiconductor, so items such as mask sets and tools that are tied to what the state Board would call a "custom made" item, are carte blanche use tax taxable. 

But, if  I can somehow link the labor charges away from fabrication or assembly of something, and tie to not to "use" but more like a prototype development effort, then I believe it's non taxable.  Sound right?  We didn't perfectly self assess use tax on some of the engineering services that was essentially labor charges on existing products, but if anyone has ideas why that shouldn't be taxed, well i'm all ears.  Thanks.