I saw a poll or two on fat pad size, we don’t have very good data on it. But if I remember right, most fat pads are between 0.5” and 1.0”. Take your 5.9” NBP plus a (roughly) average fat pad of 0.6 for an average comparable 6.5” BP.
We do have good data on BP measurements in the TP data set, and 6.5” BP is somewhere in the 55% and 71% range, depending on whether the real average BP is closer to the 5.8” that research suggests, or the 6.3” or 6.4” that is the average starting length in the TP data set.
If you believe that research shows a 5.8” average due to not bone pressing their measurements, then the 55th percentile estimate would be more appropriate. If on the other hand, you believe nearly all researchers are using BP measurements, then the 71st percentile estimate is more appropriate. Taking the mid-point makes it about the 63rd percentile.
2014 Stats Update
My 2 cents, I think most researchers are bone pressing, say 80% of them. Another 15% are NBP, and the last 5% use some other method (such as the underside measurement in the recent Indiana State sex survey). And just as much if not more error exists in the TP data and self-measurement/reporting in general.