If you've got an external, there's no need to partition the internal drive. Instead, just partition the external into a backup, Safety Clone (should you wish to use that), and data partition as appropriate.
Size is hard to suggest -- you need enough space on your backup partition to hold the whole internal drive (well, as much data as you plan to put on it). The Safety Clone/Sandbox need only be 10-12GB, and the data is what's left over.
Sound like a plan?
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--Dave Nanian
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