Half a Page of Scribbled Linux

I keep a monthly budget and have for years. This keeps me from bouncing checks and scrambling when the yearly automobile insurance is due, but I don't have a clear picture of whether I'll be playing golf every day after I retire or eating dog food. GnuCash has a retirement planning feature on its to-do list, and nothing else out there (for Linux, at least) provides this yet either. So I'm doing it using the old spreadsheet method with Gnumeric. But it's really complicated.