Not been Using xert for about 6 months. However did not go back in fitness as much as xert thinks (ftp decreased from 290 to 165 which is way too low). How do I put in reasonable values to start again (without spending time on more or less useless low traings first 
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Do you mean no data imported during last 6 months or no data with maximal efforts during that time?
There is more than one way to resolve this, but if you have known power curve numbers, I believe an easy way is to use the Power Curve & Calculator on left sidebar menu of XO.
Enter some known values, calculate, observe the power curve, then save as signature if you agree with the estimates. You only need a ballpark baseline then move forward from there with your new activities.
Couple of alternatives to @ridgerider2 in addition. I’m assuming you have data and it’s just about decay, in which case easiest are
- do a breakthrough effort to reset your signature (just ride to failure over e.g. 5 to 10 minutes… steady or on-off, Zwift race… as you like…)
- if you don’t want to do that, switch to ‘no decay’ in your profile, then go back to your last breakthrough (the one with 290W), go to advanced MPA, then save / lock your signature - from then on it will move up and down with training load. Worth testing that signature with a breakthrough at some point, but less urgent
The advantage of 2) is that if you have been training / riding, the training load calculations will be more accurate (as it’s a relative measure vs your signature)
Great, thank you!