The default tables are created and stored in a text file named tablesonly.dat. You can use the information in that file to create your own tables. For information on where this file is stored, see "Understanding Data Files" in the "Setting Your Preferences" chapter.
To edit tablesonly.dat, open it in a text editor and modify the data.
Consider the following example:
[Tables]
Count=3
1=300,200,50
2=400,500,50
3=500,500,75
If you want to just have one table that matches your actual machine's dimensions, you can set Count=1, delete all but the first table, and then type your table's dimensions.