Theres no 'master layout' as such in Filemaker.. you also cannot copy / paste an entire layout properly if you use headers / footers / summary parts etc, since these are not copied.
The only real way is to design the template and use the Duplicate Layout option each time you want to use it.
Many layout objects are intimately connected to the table occurrences, fields and scripts of a specific file. You can copy and paste layout objects from one layout to another and from file to file. You can even select, copy and paste all the objects on the layout, but if the file or layout where you paste the layout doesn't have the same table occurrence, field and script names, parts of the pasted layout (such as fields and scripted buttons) will then need to be fixed before you can use the layout.
Layout parts such as headers, subsummary parts, etc. cannot be copied and pasted. You have to create those first (Use the inspector or object Info to get the height in pixels for each part) before pasting the layout objects.
Thanks for your prompt replies. Very useful information