I just thought that when I - in the wintermonths - do something like 50 hrs on the bike and 10-12 times strength, that the strength would count as some kind of cardio/stress on the body. Which would mean that my form when shifting to eg. 60 hrs on the bike and no gym would be app. the same
Some folks do try to account for the stress of their non cycling activities, but it depends in part on whether you're more interested in having your PMC reflect cycling specific fitness or if you want to try to track all exercise induced stress and hence fatigue.
If it's the former I wouldn't include any estimates of strength training pseudo-TSS as there are very weak links between strength training and improved endurance cycling fitness especially for reasonably well trained athletes. It can make a bit more sense to track other endurance activities like moderate to high intensity nordic skiing but there's still debate about how much of that transfers to cycling fitness.
If you're more interested in tracking exercise related fatigue then you might try to estimate TSS for strength training sessions in which case I'd probably do a rough swag like an IF of say .5 to .6 per hour in the gym and use the manual workout entry dialog box to calculate the effective TSS based on hours at that rate. Among other things there's the point Fergie made that sure you might be able to estimate a rough swag at strength training TSS for fatigue tracking purposes but that won't help you estimate other sources of fatigue like poor sleep, emotional stress, long hours at the office, illness, etc. IOW if the point is trying to get a better estimate of things that might tire you out there are still many things the PMC doesn't take into account. If the point is to just estimate those 'exercise related things' that cause you to be tired then yeah perhaps you can do that via manually entered workouts and a swag at the effective IF for those activities.
One way to play with this is to make sure you enter your manual activities like strength training tagged with some other sport title than 'bike' and then maintain multiple PMCs. Use the sports filter tab on your main PMC to show only 'bike' activities and another showing all activities. Over time you can see which helps you more in terms of predicting fitness and freshness. In the end if you decide the strength training 'TSS' isn't valuable to you it's easy to just delete the 'all sports' PMC and just use the 'bike only' version.
FWIW, I don't track any kind of strength training or an evening in the rock gym or other non cardio and off bike activities in my PMC. If I do something like that I just make a mental note that I might be more fatigued than the PMC predicts and adjust my bike workouts as necessary, same as I'd do for illness or other things the PMC doesn't track.