If you are new to TrainingPeaks, or the Performance Management Chart, and do not have a history of data you may want to 'seed' the starting values of the PMC
Fitness(CTL) and Fatigue (ATL) Starting Values
A new user may not have a history of files that can be analyzed to determine their Performance Management Chart starting point and it may be necessary to “seed” the PMC with starting values for Fitness (CTL) and Fatigue (ATL)
The appropriate starting value can be estimated by realizing that most people train at an intensity resulting in 50-75 TSS per hour (i.e., average weekly IF is usually between ~0.70 and ~0.85). Those who train more, mostly or entirely outdoors, and/or in a less structured fashion would likely fall towards the lower end of this range, whereas those who train less, frequently indoors, and/or in a more structured fashion would tend to fall towards the upper end of this range.
Unless there is a specific reason to do otherwise, the same value should be assigned to both Fitness (CTL) and Fatigue (ATL).
The calculated values for Fitness (CTL), Fatigue (ATL), and Form (TSB) should be interpreted cautiously following such a “seeding” until sufficient data are available.
The default PMC constants work well for most athletes but you can further individualize them by adjusting the parameters.
Fitness (CTL)/Fatigue (ATL) Constants
Taken from Training and Racing with a Power Meter
"The default time constants of the PMC (6 weeks for CTL and 7 days for ATL) were chosen based on nominal values from the scientific literature. The precise time constant used to calculate CTL in the PMC has a limited impact. While some users may still wish to experiment with changing this value, there seems little to be gained. However, the calculations in the Performance Manager are sensitive to the time constant used to calculate ATL, and hence Form (TSB) (TSB=CTL-ATL).
Part of the art of using the Performance Manager consists of learning what time constant for ATL provides the greatest correspondence between how you actually feel or perform on a particular day and how you might be expected to feel or perform based on CTL, ATL, and TSB.
Younger individuals, those with a relatively low training load and those preparing for events that place a greater premium on sustained power output ( such as longer time trials, 24 hour mountain bike races, or long distance triathlons) often find better results using a somewhat shorter time constant, for example 4-5 days, instead of the default 7 days.
Masters age athletes, those with a relatively high training load, and those preparing for events that place a greater premium on non-sustainable power output (shorter time trials, criteriums) may obtain better results using a somewhat longer time constant than the default 7 days, for example, 10-12 days. (Since athletes preparing for longer events often, but not always carry higher overall training loads, this tends to constrain the optimal time constant more than would otherwise be the case.)”