@User i can provide 'default' values (simple equations calculating area and perimeter from W) for AD, AS, PD, PS. i need the value of the drain edge to gate , say 'Z' parameter where AD = 'W*Z', and PD='2*(W+Z)' or something similar. these 'calculated' values can be overridden by the user in the instance. This at least gives a reasonable value without requiring to fill in everything. what do you think? In the million of equations in the pdk the default values for these parameters is 0. Too optimistic. Don't know if this calculation is done internally (assuming no AD, AS, PD, PS are given on instance).