To the best of my knowledge there is no method for that. It will depend in circuit function and topology, bias current, signal excursion and the headrom you need for each transistor. Is not the same if you use cascode amplifiers than a regular one. You will find circuits operating in as low  as 100mV and James Meindl has proposed that lowest VDD value for an inverter is 36mV aprox. For transistor operating in subthreshold regime please check Yannis Tsividis books. 

The suitable threshold voltage depends on the type and components of the cmos circuit. As such no method in particular can be utilised in a way that for circuits like sram,dram etc can operate as low as 300mV and some amplifiers can work in a low VDD with 100-150 mV. Refer books for further study as mentioned by Santiago Martin Sondon

It is a good question, but in my opinion, the answer depends on the circuit kind.

Typically, the subthreshold operation for CMOS transistors is obtained by using a negative "VGS-VT" voltage. Then, the VGS voltage for subthreshold operation is dependent of VT. However, each technology has a value of VT and nowadays the CMOS processes offer two or more transistors options (standard VT, High-VT, Low-VT and native or zero-VT).   Instead of using "VGS-VT" parameter we can use the inversion level coefficient "IC" or the "gm/ID" ratio (find more information on Y. Tsividis book or EKV model). In some kind of circuits, like amplifiers, the minimum supply voltage is also dependent on the VDS voltage because the circuit should operate at the saturation to have high voltage gain. The saturation voltage (VDS) for subthreshold operation is approximately 4.K.T/q (100mV at T=300K). Then, the minimum operating voltage for a subthreshold amplifier is N x 100mV. Where N is the number of stacked transistors.