The standard depth of insulation varies depending upon the type of insulation. If your attic has spray foam for instance it may only be 3.5” and have same R-Value as an attic with 14” of blown in fiberglass. My recommendation is to check to see what the R-value per inch is for your type of insulation. You’ll want to add enough insulation on top of what you have to achieve R-40. Generally speaking 6” of insulation will have a R-20 value. This amount of insulation was common in the 70’s and 80’s. Most new homes today are built with about 15” of insulation and an R-value of R-40 to R-50. The Code minimum standard is R-34 I believe.insulation-r-value