There are several ways to cut a close 1.5 mm thread approximation. All of these methods will work, but you will need a few additional change gears. Since you have a 37/47 transposing gear we'll start there.
1. Loosen the nuts holding the spindle, idler, and gear box gears in place. Loosen the banjo, and swing the 80 tooth idler gear out of mesh with the top spindle gear. Replace your 80 tooth idler gear with the transposing gear so that the larger 47 tooth side will mesh with the gear on the lathe spindle. Replace the 20 tooth spindle gear with a 36 tooth change gear. Remove the gears from the gear box. Place the 20 tooth gear from the spindle on the gear box first. It will act as a spacer. Then place the 56 tooth gear on the gear box, and replace the nut. Hand tighten the nuts that hold the gears in place.Roll the banjo up and mesh the gear train. Be sure to leave a little play in between the gears, and tighten the everything down. The 36 tooth gear on the spindle should mesh with the 47 tooth gear on the transposing gear. The 37 tooth gear on the transposing gear should mesh with the 56 tooth gear on the gear box. Set the gear box to cut 24tpi. (left lever in the B hole working left to right, right lever in the 6th hole working left to right) That will give you a nominal 1.4997mm thread. If you don't have a 36 tooth gear you can pick one up on eBay, or someone on this forum my have one for sale.
2. Loosen the the nuts on the spindle, and idler gears. Loosen the banjo, and let it drop out of the mesh with the spindle gear. Replace the spindle gear with a 52 tooth gear. Snug up the nut on the spindle gear. swing the banjo up into place, and mesh all the gears. Leave a little play between the gears, or you will get a lot of gear noise. Tighten the banjo, and all the nuts on the gear train. Set the gear box to cut 44tpi (left gear box lever in the C hole working left to right, and the right lever in the 4th hole working left to right). That will give you nominal 1.5009mm thread.
3.Use the same procedure as number 2, but use a 54 tooth gear, and set the gear box to cut 46tpi (left gear box lever in the C hole working left to right, and the right lever in the 5th hole working left to right). That will give you a nominal 1.4909mm thread.
Since I do not have a transposing gear I have used methods 2, and 3 many times with no problems. If you don't have any of the mentioned gears you can pick them up on eBay, or someone on this forum my have one for sale.