You don't need to shorten exposure time. You needed to lengthen it because of bellows draw, and then some more for reciprocity. If you shot at infinity, there would be no need for bellows compensation. However, you shot at greater magnification than 1:1, so 2 2/3 stops more exposure were needed as compensation.
Originally Posted by Geraldine Powell
Next comes reciprocity failure. Some films handle long exposures better than others. This is a separate consideration from bellows extension, and needs to be calculated after bellows compensation - which is what the app does.
This page gives the formula for bellows compensation (see the section entitled "How Many More f/stops are Required ?"). That the app is a lot more convenient.
Reciprocity varies with brands of film, and with length of exposure. Fuji Acros requires very little compensation, even for rather long exposures. More traditional films like Tri-X or HP5+ lie at the other end of the spectrum, and require a lot of compensation. So that app is actually pretty straightforward: you enter the parameters, and it tells you the results, quite reliably, and for a great range of variables.
I recently shot some close-up images near a window in an otherwise dark room. The metered exposures were several seconds, but with bellows draw and reciprocity, the required exposures were 40 seconds. I followed the app's recommendations and things worked out satisfactorily. Here's one of those photos.
Antique Rulers, 2012
Sinar P, 210mm Macro Sironar
4x5 TMY, Diafine