Trail Boss is my go to powder in the .38-55 and a lot of other cartridges.
I can't imagine how you would double charge it. It was designed specifically to avoid that. It is true, it doesn't like to be compressed. Otherwise, you can fill the case to the base of the bullet without issue. I get best result in any cartridge about 1/10th of an inch below the seated bullet. The exact charge will vary depending on what length brass you are using in your .38-55.
Here is a pdf. from IMR outlining how to use it with any cartridge.
http://www.imrpowder.com/PDF/Trail-Boss-data.pdf
I can't imagine how you would double charge it. It was designed specifically to avoid that. It is true, it doesn't like to be compressed. Otherwise, you can fill the case to the base of the bullet without issue. I get best result in any cartridge about 1/10th of an inch below the seated bullet. The exact charge will vary depending on what length brass you are using in your .38-55.
Here is a pdf. from IMR outlining how to use it with any cartridge.
http://www.imrpowder.com/PDF/Trail-Boss-data.pdf