Answers in reverse order :

Rough position bail : (Mast length - hound height) * 0.6 + hound height

Never go beyond that. If you are really scared that the mast won't hold than use 0.5 instead of 0.6 This halves the bending on the mast. Whatever you do don't believe Bill Roberts reply. He will have you put it 0.33 above the hound which is way to low, the mast can take alot more especially when it is a modern cut spi.

Get the rope spi bail. There is NO reason to fit that metal thing up there. Really no advantage what so ever.

Patches in spi, there is actually a way to discover it yourself.

Lay the spi out on the floor. Grap the spi in the points you think are about right and place these all in one hand. Now lift the spi and stand on a chair or something. have somebody else straighten out the spi towards the floor. If one part of the spi sticks out below from the rest of the spi try to determine which part is it (often it is the top or the tack or part of the leech) and lay the spi on floor and move the nearest point towards the part that is sticking out. Do this a few times and you'll get it right. Then mark the spots and make the patches there.

It helps if you use small pieces of tapes to mark all the spots that you have held at a time.

There are formula's to do this but really just doing is both easier to explain and easier to do and certainly more dependable

Wouter