Well let me give you a very approximate formula based on the July-August orbit. The distance to the Moon is given by D = 6378/sin(theta) where at 5 day intervals:
Day.....Theta(arc minutes).....distance(km) July 1 61 359460 July 5 59.9 366000 July 10 55.9 392200 July 15 54.0 406000 July 20 54.6 401590 July 25 57.9 378700 July 30 61.4 357100 Aug 4 58.5 374800 Aug 9 54.7 400800 Aug 14 54.0 406000 Aug 18 55.2 397200 Aug 23 58.5 374800 Aug 28 61.1 358800 ........................................Example: on July 20, 1996 Theta = 54.6' = 0.91 degrees. sin(0.91) = 0.0159 and so D = 401590 km or 257,018 miles. Since the orbit is a periodic function with a period of 29 days, you should be able to create a simple periodic function of the form:
D = A + B cos( 360*(T/29) )where T is the number of days since one of the days in the above table. A bit of trial and error will get you a solution for A and B when you plot the distance and dates above.