What the instructions are trying to do is to make the angle about 45 degrees to minimize the effect of elevation angle measurement errors. If you are off by +/- 1 degree in your elevation angle measurement at 45 degrees, your calculated altitude will be in error by only 3.5%, and within 4% for elevation angles between 30 degrees and 60%, and within 5% from angles of 22 degrees to 68%. See the attached plot.
If your flying to 250', then the base is ideally 250'. If your flying to 500', then the base is ideally 500'. If your flying to 1000', then the base is ideally 1000'. Etc.
The altitude for an 65 degree elevation angle with a 250' baseline is 250' x tangent (65) = 250' x 2.1445 = 536' +/- 4.56% = 536 ' +/- 24.4' error for each degree of elevation angle error.
With a 500' baseline and the same altitude the angle becomes arctan(536/500) = 47 degrees and the measurement error would be reduced to 3.5% or +/-18.4' per degree of elevation error. For fun flying it not that important, but for competition it is.
Bob