I can't find the course design guidance notes on the BTA website.
However these are key paragraphs from our own regional notes:
... makes it important that all league courses should ideally be laid out to a standard or normal level of difficulty.
A course of 32 freestyle targets at 55 yards plus eight positionals at 45 yards would technically be allowed. However, it is hoped that course designers can see that this is quite likely to be unpopular.
Likewise, a course with nine 1 inch apertures at 45 yards and seven ½ inch apertures at 25 yards would also be technically legal but could lead to complaints.
Course designers are asked to recognise that most shooters simply want to have an enjoyable day's shooting, with a fair chance of achieving a score that is appropriate to their grade and position. The rules provide enough flexibility to allow a wide variation of course layout from venue to venue yet at the same time achieve a reasonably consistent level of challenge.