RF=312[MΩ]
LD=22.9[cm]
LT=0.9[cm]
IF・RF = E・LD	---(1)
E・LT + VGUARD= IF・RFT	---(2)
VC = IF・(RF + RFT)	---(3)
VGUARD=Vm	---(4)

From (1),(2),
RFT = (LT + VGUARD/E)・(RF/LD)	---(4)
From (3),(4),
VC = (RF + RFT)・(E・LD)/RF