As you describe it, and understanding the way that club face angle and loft are related, if the loft is 18.7 with a face angle of 2 degrees open, the “effective loft” or loft at impact (assuming square at impact) will be 16.7. I know, in looking at some of TM’s charts, that on a driver if I have a 10.5 degree driver and change the setting from square to 2 degrees open, THEY show the loft changes to 9.75 degrees. The lie also changes, but with regards to the loft, it does go down, but just 3/4 of a degree, not 2 degrees. This tells me that if you measure the head in the square position with the square hosel setting and the loft is 10.5, then you change the hosel setting to 2 open and measure the loft again, you have to measuring the loft with the face angle back in the square position to get the reduced loft. On your 5 wood, my assumption would be the same, but due to other angles involved, the 18.7 might not be 16.7, but somewhere in between. Might be worth a call or e-mail to TM to get their take.