A way to have slightly better numbers with GWT is to filter the data showing only web search since image search also affects in terms of impressions and average rankings. However numbers are still so approximate that I haven't bothered to merge them with GA. If your purpose is to disclose the (not provided) what I do is to calculate the % of total and then add that % to the traffic of each shown keyword since I assume that loggedin users behave in the same way of not logged in. If your purpose is to see rankings, I think that there are plenty of tools out there better than GWt to track them. HOpe this helps!