I carry a Secumar and fitted the block as high as I could manage – it is just a couple of inches below the top of the mast. The reasoning was that you’d like as much leverage as possible if it does fire. I bought the smaller of the two secumars, to avoid putting too much weight aloft; it has worked once in practice in quite rough water.
See other posts in this forum – there has been a discussion about the secumar flapping about; folk proposed varous solutions, including fixing it top and bottom. I tie a loop on the bottom of a secumar through which I run the secumar halyard returning down the mast.