Anyone has an accurate methodology to calculate the ROI of the skills trainings?

Cost and benefit analysis.

I think that the cost and benefit analysis will be more accurate with the vocational or the technical trainings but can I challenge you and ask you if you delivered a communication skills training to the finance team inside your company how your are going to calculate the ROI using the cost and benefit methodology?

Even that calculations is not too much fair for assessing training's  ROI ,

cost benefits analysis may bring a light and should be done 

  ROI =Net program benefits/program cost in other words its the earning divided by Investment. The    analysis accurately based on the nature of project , you can get more enough details jack Philips reference "The Value of Learning". I'm ready to support .

Investment return measured over the interest earned by the employee and the impact on the organization through actions by the terms of time and achievement at work