Computing K parameter for Mosfet/JFet/
K is called the conduction parameter. The simplest way it to get it from the SPICE model that the manufacturer gives for the FET. If the manufacturer doesn't give a SPICE model for a FET... it's usually "no sale" for me...

If you still want to get it from a datasheet... you need to do curve fitting as described in MOS Model Parameter Extraction Techniques: A Comparison
by C.G. Cahill, K. McCarthy, W.A. Lane [for instance]. There are some youtube videos showing how to do something like that with just two data points from the graph, but the error you'll get that way is going to be large.

There's a on-line app that seemingly does that (properly) but it's unfortunately in German. However that should give you an idea what you could; essentially ask the use to input several data points from the FET characteristic... the more the better in terms of accuracy.