In fact any position on top of this point is safe (tensioning the solent stay you only get compresion on the mast and tension on the back stay)
Any position bellow that point will also generate a flexing force on the mast.
How far bellow the point can you go? Depends on the section of the aluminum
mast. The lower you go, the bigger flexion you get. At some point, you will need to add running back stays to control flexion (we will not be talking about solent stay any more).
There are other very important considerations about solent stays:
The reason for the solent stay is to hoist a small headsail without unrolling and changing a big genoa
while the weather
is building up. So you need to to define the position of the top, clue and tack corners of the sail.
First, find a strong place to fix the tack corner (as recommended by Pblais), you may need to add a strong chainplate fixed to a structural
bulkhead in the front or the back of your chain locker (special care must be taken if your deck is a sandwich structure). Check that the Highfield Lever you are going to use (to fix/remove the stay) fits the chainplate.
Knowing the position of this point you can apply the procedure described in my previous post to determine the top attachment of the solent stay (remember that the foresail halyard
exits the mast quite bellow the forestay top attachment to avoid jamming the halyard
Finally (and very important) if the tack corner is not very close forward to the forestay you will need also to position 2 strong fix points (or two little rails) to hold the sheets
blocks of the solent sail. I guess that the position of this points should be 12/15 degrees from the centerline starting at the tack corner. (this angle is a first guess from my part: you should check the this angle measuring the one you already have in the foresail of your boat).
If the tack corner is close to the forestay, you can use the current genoa
rail for the sheets
(with the car in the forward position). Otherwise the angle will be too big to sail close hauled with the solent sail.
I'll like to make clear that I'm neither a proffesional rigger nor a naval architect, I am just walking your same steps and this recomendations are what I already found out on the subject.
Hope you understand my english
(it's not my mother language), and this thoughts helps you in your project