spent a month at -50 or below when i was cycling 20-25hrs a week in france for a month vacation. FTP was probably correct too, because there were plenty of 1hr+ climbs to test myself on. first 2.5w were fine and then i hit my 'peak' of -76 during and was pretty miserable for a couple days until i returned to around -60 for the last week. i remember googling 'how can you tell you are overtraining'? pretty much every day during that month because up until i cracked my legs had felt fine despite the hugely negative TSB (i've now come to realize that if you have to ask if youre overtraining you probably are)
definitely wouldnt recommend it for optimal training though. so far this year i've mostly hovered between -15 and +10 which seems better. i think the reason I was able to go so negative was mostly because i was only at around 50 CTL before the trip and i'm pretty comfortable sustaining 110-120 CTL (which is where I currently am). I couldn't go -76 TSB on my current training load.
do you guys actually find TSB useful? i wish i could use it to predict when my legs would feel good or not, but it doesnt really seem to do that.