Set it for 47k ohms, that should do the trick.
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There are 2 issues involved in your decision: 1)Input load - set it at 47k. Tt doesn't make a difference if it is MM or MC, that is just loading not gain. 2)Phono stage gain - set it on the low setting. MM cartridges are (at least so far as I know) much higher in output and need a lot less gain from your phono stage. Check your Grado Spec's and compare them to the Shelters spec's for gain and the difference will give you a clue.
The 47pf is for loading for Cartridges which need it - most typical MM cartridges, and you select values by calculating the total picofards needed including the IC's to match the cartridge with the phono pre. Grado's and most MC's do not need resistance loading so you can ignor it for these two cartridges.
But do set the gain of the phono pre low. If I'm wrong you've lost nothing except a few minutes necessary to increase it.