It depends... if you are using standard source coupled pair with/without R degeneration to linearise your input then it will obey the  10dB rule more or less and the third order intermodulation distortion will grow fairly linearly at 3dB/1dB until one nears compression. So in this case I only use V1dB as its usually much quicker to simulate and yes V1dB*3.16 will give IIP3.

IF however you have a differnent linear gm structure, then the 10dB rule might not fit, ESPECIALLY under specific signal levels (which is often the case in a well defined system)... a classic example is using the multi-tanh (offset source-coupled pairs) to linearise your input.... in this gm structure the IIP3 is not simply something growing 3dB for 1dB input increase in a linear fashion (although overall trend is correct), it follows a harmonic signature which has growth gradient which varies drastically (ie not 3) depending on absolute input signal level.... infact it also has third order intermodulation minima as well... now in this case you cannot just assume that the IIP3 is 3.16*V1dB at any given V1dB. Please google Gilbert's classic multi-tanh princple to see this specific example of why 3.16*V1dB assumption might be dangerous.

So what I would do is determine in one simulation the IIP3 and V1dB swept over input power to confirm the harmonic signature, then when optimising linearity, you only need to consider V1dB, before finally returning to an IIP3 simulation to confirm you meet your spec.

the IIP3 seeming to be very very dependant on extrapolation point means one of two things:
1) your gm structure has a non 3dB/1dB third-order intermodulation signature OR
2) your simulation setup is not configured correctly
doing a swpet IIP3 allows you to see whether you can see "the slope of 3" and chose your own extrapolation point to be slap bang in the middle of it.

You should take a look at http://www.designers-guide.org/Analysis/intercept-point.pdf.

-Ken

while simulating  iip3 for my folded mixer, i found the third order intermodulation curve is not giving slope 3 any where.
it is varying between -5 to 2.4.

what might be the root cause for it??
i cant take the region having slope -3 to fix my extrapolation point.

and one more thing is that iam getting a cp1 of -37dbm and the iip3 0.72dbm( if i take the region having slope 2.4 for my extrapolation point). The difference is huge...

can any one explain where i am doing the fouls???

thank you.......