I once made a spreadsheet garden schedule like you're describing. Was a fun exercise but a couple years hands-on experience in the garden made me realize it was useless for forecasting anything.
The seed packets are grown, packed, and distributed nationwide. The farm the seeds came from is likely in a totally different climate than my garden. So when the seed packet says "50 days to maturity" - 50 of which days? The day length in April when I start planting is 13 hours, by the end of June a day is 15 hours long, and in September/October when the bulk of my harvest is coming in, a day is down to 10 hours.
So just the variation is day length accounts for a +/- 20% tolerance. Which would mean a seed packet would have to list "40-60 days to maturity" which isn't even precise enough to be useful.
So I now ignore the DTM on the seed packet and simply group plants by planting date: spring/summer/fall, and season length - i.e full-season crop or succession crop. For example corn, brussels sprouts, winter squash all take a full season for me, so I plan for those beds to be occupied. Beets, carrots, green beans are half-season crops for me, meaning I can plan to turnover the bed once mid-season. That's all the precision I need. The vagaries of weather will determine exactly when each crop is ready to pick.
The other useful habit I got into was a garden journal - I use a note-taking app on my phone, but paper and pen work fine too. I take pictures and write brief comments. It's interesting to look back and note what a crop looked like on this date last year and two years ago. Sometime despite having "the coldest April on record" followed by "the driest May in history", the garlic puttered right along and I harvested it on the same weekend I did last year!