hi there,
Nerd alert 
i understand that the fitness value is calculated by the latest 42 days TSS average.
if i want to calculate how much TSS i will need for my training tomorrow in order to simply Keep my current fitness level would this be correct?
F=current fitness
TSS42= target TSS for the next day
TSS(n)=TSS(day 1), TSS (day 2), etc
42 = Total days considered for the average
F=(TSS1+TSS2+…TSS42)/42
in order to keep F constant for the next day (considering now that the TSS days numbers should be considered one day later, so the TS42 is what we want to know.
TSS42= (42+F)/(TSS1+TSS2+…TSS41)
would this be correct or rubbish?
thanks 
It’s a ‘rubbish’ question because Fitness should not be a goal. Fitness has to be used as a planning tool, to help you build in a sustainable way.
If you want to know what the impact of future training is on the fitness number, just plan workouts ahead with a TSS number and use the feature to project Fitness in the future.
Well, I don’t want to use as a goal but understand how is the minimum effort in a recovery ride for example, to only keep the fitness (and Imean the number using the CTL).
I saw also a relatively old guy that wanted to keep his fitness and that was his goal as probably he wouldn’t make too much progress from where he was. So, for some people’s ,keeping the actual fitness could be a goal and understanding how much effort they should be adding each week is important in my view.
Cheers
What you want to keep is ‘performance’ and it’s not the same as fitness.
If you want to keep the fitness number, it’s really easy: just do exactly the same thing week after week. Same amount of time, same intensity. It will result in rock stable fitness. And the best thing is, that performance might even go up.
Everyone reaches a Fitness plateau sooner or later and the height of that plateau is strongly correlated to exercise time/volume.
And that is absolutely wrong. The purpose of a recovery ride is to promote recovery. If you set TSS goals to a recovery ride, you’re training and not recovering. Thus you miss the whole point of a recovery ride.
i agree, but my point was if i understand how much TSS i would need for my next ride in order to keep the fitness number as it is, i could use a recovery ride if this TSS is relatively low. obvioulsy, if its high, it doesnt make any sense as you said.