@User an interesting option is to put the correct equations in the instance. Once this is done for one instance use this one (by copying / copy/paste) anywhere in the design. Changing W,L, nf will show the correct calculated values for AS,AD, PS, PD, RS, RD, so you don't have to do calculations whenever changing L, W, nf. See sample image. Another possibility (that does not exclude this one) is to change the equations in the symbol definition, so when placed for the first time it will already have the correct formulas. From a thread in #analog-design