At a certain point, I stopped using silvers because the 'known formula' stopped working for me.
(And by the way, I never tried to be chintzy with the Goddess: I always dropped at the multiple of 5 [1605], rather than cheaping out at just +1 [1601].)
Try using gems instead.
The good news is, from what I recall the last time I stocked up, the value of the gem (as it appraises) can be significantly lower than what the silver formula calculation would have you dropping. The gswiki page seems to indicate "roughly 1/3 the silver." (i.e. appraise your gem and triple the result; that's what your deed contribution would come to.)
The bad news is this entire thread may well be pulled, as revealing game secrets.
