NMOS characterization in 0.25um Technology

Hello guys,
I am trying to characterize an NMOS transistor in 0.25um tech node to aid quick hand calculation in my design. My first task was to generate Vto (threshold voltage for zero source bulk bias). So i generated the I-V curve for the NMOS with W=10um and L=0.35um (I chose L > min, so i can have channel length modulation effect). so i read the current values for two different curves ( VgsA , VgsB) at a certain Vds and ensuring they were in saturation. So i used the formula Ids = Kn/2*(W/L)(vgs-Vto)^2*(1+λVds) and found the ratio for the Ids of the two curves and solved for Vto using above mentioned equation and current value measured. My problem is, i am getting a negative number around -0.5 volts for Vto, which is wrong because its an NMOS, and i checked the value in the spice model file and it around 0.366 v. I don't know what i am doing wrong.

Did you check that you have to take square root of Vgs-Vto to get Vto.But you have to neglect one value of the two values.

Generally, maximum gm method is used to decide Vth.

Yeah I have the same question, so what's this "maximum gm method"? Where can I read about it?
Thanks!

Maximum gm point can be found in your I/V curve where dI/dV is the maximum.