I hope you don't mind if I ask a couple more questions?
Have I ever minded?

First, how and when "Iststarke" was introduced in the German reporting system? Were there any directives or instructions from, for example, the General Staff or its departments? The second: how were wounded and sick and medical facilities of armies and army groups registered, were they included in Iststake figures?
It was a well-established strength term (I didn't know it actually dated back to WW I, thanks Michate), but during the early part of the war it seems to have just been used to determine the exact manpower strength of the Wehrmacht once a year. Besides by the medical officers, in the
Truppenkrankennachweise. Its definition appears to have been in essence unchanging (the only change as far as I know being that from autumn 1943 it included sick and wounded to return within 8 weeks, rather than 6 as previously). There must certainly have been instructions, but I have never come across a document that accounts clearly for why it starts appearing fairly regularly from mid-1942. The definition of it can be found lots of places though. Second question: It wasn't a question of where, but when.

As long as the soldier was expected to return within 6 (later 8) weeks, he stayed on the Iststärke. At least in principle. There was also a change in practice in fall 1943. Prior to that time, soldiers in Heeresgebiet (ie, army group) hospitals were evidently taken out of Iststärke if they were not initially expected to return within 6 weeks. According to this order, they would henceforth only be taken out of the Iststärke if and when they had spent 8 weeks in hospital without yet being recovered. You will appreciate the significance of this for the resulting figures. The reason why this was important was that going off Iststärke also meant that you passed out of the Field Army and ceased to belong to your unit for the duration of your convalescence. In principle, you would then return to your unit (and administratively, to the Field Army) as part of a Genesene-Marscheinheit. Once that happened however, there was a significant chance of being diverted elsewhere, as
Genesene were often drawn upon for experienced cadres for new formations and so on. As long as you were on the Ist, you still belonged to your unit, and return to it was automatic.
when did this cange occured and why?
Good questions. I wish I was able to answer them better than I can. Mid-42, to judge from the reports you see. But it doesn't just suddenly start appearing everywhere, there is still a lot of variation. Since I don't have a document explaining how and why, I can only speculate, but the obvious reason seems to me that it was an already familiar strength definition that filled a need that wasn't met by the previously common Verpflegungsstärke and Gefechtsstärke. Both in the sense that it provided a better and more reliable measure of general strength than either of those two (although it was far from ideal in that regard), and also in the sense that it responded (in this case more or less ideally) to the needs of manpower management. In essence, it provides the number of soldiers belonging to the unit, whether they are present or not.
cheers