Your concept is correct !
Once the line is marked I then set up my tripod with the leg marked “N” on the top base pointing roughly towards south in this line
Your tripod then must be levelled as best as you can in all directions
Then I use 2 x 1 metre lengths of 30 x 30mm aluminium angle , first one sits on the ground along the new true south line with the end just touching the inside centre of the tripod leg, move the tripod leg to line up with the aluminium angle and second runs across the 2 other ( east and west legs ) tripod legs or feet at the bottom
I then use a builders right angle square to align the right angle formed by the 2 lengths of aluminium until it’s exactly at right angles with no gap along the south facing aluminium
Once again do this as accurately as possible
The tripod is now set up facing True South
Sorry it’s a bit confusing
I’ll try to send a diagram
Hopefully you get the idea
