I am not finding how to make a new base plan in Help, but I thought for sure it was there.
To Make a Base Plan.
Open the project that has a project plan that you wish to share as a base plan.
Open File menu, then Project Plan
Holding down the shift key click the Manage Plans button
Make any other changes to the plan
Click on the Save as a Base Plan button
Give the plan a unique name
Save.
The XML file with the name you just gave it is will be in the directory c:\MyParatext8projects_standardPlans. Go there and copy that file.
Copy the file into c:\MyParatext8projects_standardPlans directory on the computers that need to use the plan.
The administrator for each project will need to choose the new base plan for their projects