My tip for down tube name placement on classic era frames is that it is usually located between the top water bottle boss and shift lever or derailleur cable stop (unless it is a very long name). Otherwise when you are looking at it from the drive side the lettering looks too close to the big chainring. Also there is a tendency for people that haven't thought about decal placement to put the name on the very side of the frame. However one looks down on an assembled bicycle so the decals need to be rotated up so one can read the whole name without kneeing down. This means that the space between the tops of the letters is fairly close together and there is a lot of space underneath the frame between the bottom of the letters. How rotated depends on the size of the letters but 3/8ths of an inch is a rough approximation. That distance varies deeding on the style of font used and whether it has upper and/or lower case letters.

This rotation also applies for any letting on the top tube too. Another rough rule of thumb is that the bottom of the letters are usually right on the side middle of the tube (unless they are fairly better letters). It is easy to find the very side of a tube using tape that when pressed against leaves a mark. I cut a piece of semi translucent tape and place it crosswise on the top tube near the head tube. I take a metal straight edge (usually a ruler) and make sure the edges are touching both the top and down tube at the same time. When I run the ruler over the tape it will leave a mark right on the side. I then place my aluminum angle along the tube tube using my newly created mark as a rotation guide. The angle piece serves as a guide to place masking tape along the tube in a perfectly straight line. The masking tape of course is the guide for placing the decals.