Prism says my user-defined equation is "too complex".

"Too complex" means that the equation has too many operations. Prism compiles your equation into an internal format it uses to compute it, but your equation won't fit in the space Prism sets aside for this purpose. Each version of Prism has alloed more complex equations than did the previous version, so upgrading may solve the problem for you.

You can usually rewrite an equation to make it less complex. Do this by defining an intermediate variable that defines combinations of variables. For example if your equation uses the term "K1+K2" four times, you reduce complexity (but keep exactly the same mathematical meaning) by defining an intermediate variable at the top of your equation (say, K1K2=K1+K2) and then using that intermediate later in the equation. That way Prism has fewer steps to store.

Also avoid calculations within the equation, when you can. This syntax (X^(1/2)) is valid but requires two operations (first divide 1 by 2, then take X to that power). Instead use this syntax (X^0.5) or this (sqrt(X)).