OK I am digging a bit deeper and note
from :
the “one-sixth rule” places the first standard parallel at one-sixth the range above the southern boundary and the second standard parallel minus one-sixth the range below the northern limit (figure below). There are other possible approaches.
also:
The standard parallels are usually chosen at 1/6 and 5/6 of the latitude range of the map.
As regards the standard parallels used by aero charts, this seems to answer the question from the
jeppesen website (my bold):
In order to support the European wide standard, a Lambert Conformal Conic Projection with Standard Parallels at N37° and N65° is used for production of all VFR+GPS charts. This enables the seamless combination of all charts and the use for continuous flight planning. A scale distortion is a consequence of the projection and varies with the distance from the Standard Parallels. The true scale is depicted on every VFR+GPS chart on the Cover panel (e.g. EB/EH: True Scale at N51° - 1:515.000) and near the Scale Bar. In addition, a factor is given to multiply measured distances with every parallel (e.g. 1.03).
Is this also the case for the ICAO charts (I cannot find a similar scale multiplier on the chart at which I am looking)...?
Does anyone have any idea of the worst case scale accuracy for such a 1/6, 5/6 projection ?
hugel