Not immediately a formula to put in a spreadsheet but this is a readable format of the calculation:
Today’s CTL = Yesterday’s CTL + (Today’s TSS - Yesterday’s CTL)/Time Constant
where Time Constant is in days. Default values of 42 days for CTL and 3-10 days for ATL (7 is used as a default)
ATL is same formula as CTL, simply use ATL instead and the relevant TC.
Today’s TSB = Yesterday’s CTL - Yesterday’s ATL
Don’t forget that to calculate your needed daily load to remain in the green will need a recalculation every day because your CTL changes everyday…
A TSS of 60 has a different outcome if your CTL is 40 or 120.
Found a reputable source:
https://help.trainingpeaks.com/hc/en-us/sections/205644448-Glossary-of-Terms