Back to first principles:
The Moon and the Earth are illuminated by the same source and the distance from the source is approximately the same.
The albedo of the Moon [surface reflection] is approximately 0.5.
Therefore the exposure is one f/number smaller [read bigger aperature] for a full moon. Two f/numbers for a half moon. Interpolate for the rest of the phases. The shutter speed is 1/ASA film speed.
ASA 100 => 1/100 second [1/125 second]
ASA 400 => 1/400 second [1/500 second]
For a telescope with an f/10 aperature and ASA 100 the exposure would be:
Full moon => 1/125 second
Half moon => 1/60 second
Also the Moon moves 15 degrees per hour. For a 50mm lens on a 35mm camera the longest exposure without motion blurring is 1/25 of a second or 2*1/focal length in sconds. For a 6x6 MF camera an 80mm lens would also be 1/25 of a second. The tradeoff that arises from using a telescope is that the longer the focal length of the telescope, the shorter the exposure must be EXCEPT THAT an Equatorial motor mount will take this motion out.
*** John, are you using an Equatorial motor mount to eliminate the relative motion of the Moon? ***