The next steps will be to make a 'square in square block' using the 4 patch as the center square. In order for this unit to come out a specific size the center square usually measures at an eighth measurement. Here's some examples:
http://quiltville.com/sqinsqchartpf.html
If you want your Snail Trail to come out 12.5 unfinished, that's what you need to cut.