It really depends on the balance/suitedness stats of the weapon, in addition to the weight factor. Undoubtedly your effort to reach lower RTs with a greatsword will pretty heavily favor bumping up strength over agility though.

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Ogdaro
"Take chances and see what you can get away with, it only costs you a favor or two if you mess up." -Issus

At 102 stones, you need 66 as the weighted average of your strength and agility. Given that balance/suitedness, I'd guess the weighting is roughly 80% strength, 20% agility. So you'll need ~12 more points of strength.