That's where the reality check that Pilot DAR mentioned needs to come in. Any computer based program can have errors - the old garbage in, garbage out adage applies.
You don't program the spreadsheet and go. You run a lot of different scenarios. Ones that will not cause issues and ones that you know before you even enter the numbers would never work. You then compare all that against the POH or AFM. You get comfortable enough that you know it will work when properly used.