but I'm tall (6'4), and I made my hammock longer, so I went with 15x10, before hemming.
If the 15' is your ridgeline, it will/can be long enough to interfer with your hammock suspension. (In most cases)
I'm 6-4 and find a 10' hammock fine for comfort. Yes bigger longer hammocks do allow for a flatter lay, but they add to the bulk of the pack. Double-edged sword thing. Longer the hammock, the longer your tarp needs to be.
10' hammock with a 101" ridgeline is fine for most. The same basic dimensions are in many popular vendor hammocks.
First attempt: Big bucket, some mineral oil, tube of silicon,
I'm hoping thats a typo..mineral oil and MINERAL SPIRITS are two different things. Use mineral spirits. (I see later in your post you said mineral spirits, so I just wanted to clarify
.... be spoiling the tarp by waterproofing it multiple times. I can't see how. Any material is going to remelt(? that can't be the right term) the silicon in the tarp, then deposit some more.
I think it may add a few ounces to the tarp. Each time you dunk/coat/paint the solution on your leaving a thin layer of silicone behind once the mineral spirits evaporate.