What am I missing on fitness calculation?

[Gerald]

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Calculate Needed Training Load?

I have been trying to keep myself in the green optimal zone for most of December. For me so far though, it’s been a bit of trial and error. I do a Zwift workout and I fall into grey. The next day I add an extra 10 minutes and I squeeze back into green. Is there any way of calculating or estimating how much work I need to do in any given day to keep myself at a given level? I just feel like I’m bouncing around an awful lot.