That page you pulled stats from, http://www7b.biglobe.ne.jp/~fit/memo/st.htm) has a different formula for the effect of AGI on monster TP gain. They use:
ΔAGI = 自AGI - 敵AGI
AGI枠モクシャ = 15 + int( ΔAGI / 2 )
ΔAGI = +70のとき、モクシャ50(上限)
ΔAGI = +00のとき、モクシャ15
ΔAGI = -30のとき、モクシャ0?(下限
dAGI= Player AGI- Monster AGI
AGI subtle blow effect= 15 + floor( dAGI / 2 )
dAGI= +70 when mob TP gain is 50%(50% subtle blow)
dAGI= +00 when mob TP gain is 85%(15% subtle blow)
dAGI= -30 when mob TP gain is 100%?(0% subtle blow?)
They test AGI by casting a spell 10 times and charming, which obviously only works for charmable monsters.