I apologize as I don’t know the exact terms Strava uses, but Intervals.icu and Training Peaks (I think), use fitness, fatigue and form, so I will use those. I think it is easier if your ‘fitness’ is lower to get a more negative ‘form’. form = fitness - fatigue. Say your ‘fitness’ is 20, then you only need a 60 ‘fatigue’ to get the -40 ‘form’. If your ‘fitness’ is 100 you need to hit a 140 ‘fatigue’. I guess at the unrealistic end, a ‘fitness’ of 0 makes any kind of training take your ‘form’ drop below zero and would be the easiest.
Personally, I cannot stand the fitness / fatigue / form nomenclature. I understand why it came to be, namely because of trademarks on CTL/ATL/TSB, but I think it really confuses the topic. My perspective is that a high chronic training load does not mean you are ‘fit’, it means you have a high chronic training load (relative to your FTP.) Does having a ‘fitness’ of 100 with a 200 watt FTP mean person A is ‘fitter’ than person B with a ‘fitness’ of 80 and a FTP of 300 watts (all other things equal)? I don’t think so, I think it means person A has a higher relative chronic training load that person B.
I tend not to have TSB/‘form’ drop below -20 - -25 at the most. My perspective is that if it is too negative it means I am ramping up the training load too quickly relative to my long-term load, and that is usually not sustainable for me and can lead to overreaching (TSB = CTL - ATL.). A training camp like week might get me below -25 there but I am not able to do those often. I can’t imagine how my body would feel if I hit -40 or below.