The rules of evidence (FRE 704) allow opinion testimony in civil cases, whether lay or expert, to embrace the ultimate issue in a case. (See Westfield Ins. Co. v. JCPenney 466 F. Supp. 2d 1086, 2006 (W.D. Wis). But it is discretionary and there are limits. For example, an expert cannot offer an opinion that amounts to a legal opinion (See Anderson v. Suiters 499 F. 3d 1228, (10th Cir. 2007) or is a bar conclusion lacking supporting data. (Tabatchnick v. G.D. Sear & Co. F.R.D. 69 D.N.J. (1975)).
The best use of your expert is to teach the jury. Use simple words and exhibits to explain the expert’s testimony.