Sowing calendars

I guess a lot of it depends on the importance of getting sowing and cropping right, in other words, is your home grown produce what feeds you or a really nice addition to what you buy? When I started out we had to live off the land so I couldn't leave it to chance.
I still use my spreadsheet and improve or develop it year by year, but there is far more blagging it, especially later in the season. In spring it is the bible!!