OK. I corrected the statement to Doug's suggestion:
UNEstNtWt: Switch([tblFGUOM].[UOMType]="WT",Round([UNNtWt]*0.05+
[UNNtWt],6),[tblFGUOM]*.[UOMType]="VOL",Round([UNNtWt],6))

This is behaving. Obviously, my "or" setup was faulty.

Now I'm back to square one! Query too complex / System resource
exceeded. Simplify to this and all is good:
UNEstNtWt: Round([UNNtWt]*0.05+[UNNtWt],6)

Sorry for all of this craziness. I'm trying to work it out but not
doing so well. I'll leave it here and wait for any replies.