Just to add to this, it is actually rare in our experience that this happens. Now there is always margin for error but especially with 3M, they are usually on the ball with their quality. We have been a 3M certified shop for the last 35+ years. This is just our experience though.
With how many of the 14 gen's that we have had go through the shop, I would just do 20% all the way around. Depending on what the exact shops 20% film meters at. You could be looking at anywhere from a 17% - 23% usually. Laying that over the back factory tint gets it closer to a 8-10% so its not...