After a hiatus due to real work getting in the way, I finally found time to re-work the replacement left horizontal stabiliser forward spar.
A little bit of deja-vu working on this, naturally, but the result this time is far better. In fact, the line I drew along the centre of the flange lined up nicely with the skin holes at first try.
As you can see in the photo below, the lower spar flange (new one) has a much moe satisfactory edge distance than the (upper) original.