Hi John,
The error in calibration for a given altitude difference depends on altitude. The best match would be to the mid point altitude but in practise except at low altitudes the error is small for 5-10 deg difference in altitude. For more details see my recent post here
https://britastro.org/comment/5536#comment-5536
The ARAS forum link there gives links to Francois Teyssier’s spreadsheet for choosing reference stars which includes MILES and other potential stars and takes the error into account. There is also a chart linked from the ARAS post showing error relative to altitude.
Cheers
Robin