I think you are right to question the efficiency of the triangular gauges.
From bitter experience I know that the outcomes are not reliable; well not for me at any rate.
I suspect that the inline slots on the outer studs (on the outside of a curve) may tend to flatten the rail between them instead of curving it out, giving rise to gauge narrowing.
I discovered 50%+ of my poitwork was narrow to gauge and I reckon I relied too much on these gauges.
Now, whenever I build pointwork, I always use a template, usually from Templot, and deliberately build slightly over-gauge.
Then I check everything with a vernier.
Slightly over-gauge at turnouts is desirable; it allows extra clearance for point blade fitting to stock rails in particular.
As I use plastic chairs on ply sleepers, it is fairly easy to apply solvent and move a rail out slightly if narrow to gauge.
Also be aware that if you use these gauges on plain track with C&L chairs the gauges will tend to grip the rails at their base.
After removing the gauges the rail will then move to conform to the 1 in 20 cant of the chairs and you end up with gauge narrowing!
One answer is to file the gauge studs down so they only grip the rail head.