yes, the accuracy tests were done at 100 m from a service rifle which was mounted in a fixture (not fired from the shoulder).
There even existed a specification TL 9904 “Prüfvorschrift für Schusswaffen 98 als Beschusswaffe” that laid out the requirements for a service rifle to be used as test weapon. Alas, I never encountered a TL 9904 (TL = Technische Lieferbedingungen, equivalent to a military specification). “Schusswaffen 98” is a global term for the weapons firing the 7.9 mm ammunition.
The dispersion data I cited (valid for S and sS) are taken from TL 13/1001 dated 03 Nov 1932, change status 14 Sep 1940: Vorläufige technische Lieferbedingungen für Geschosse S., s.S. und S.m.K.
SmK requirements were: average group size 16 cm, none larger than 20 cm.
SmK L’Spur requirements (TL 13/1002, title sheet missing, dated 22 Dec 1939): average group size 20 cm, none larger than 24 cm.
Hope this is useful to you. To check the above values, a transparent template was used that presumably showed circles of the required diameters. Drawing number was 13E9202 L25, but I never saw one.
P.S. If you anyone knows details of the procedure to obtain R50 or R100 values in the Warsaw Pact armies (especially what was done if a lot in first test failed the requirement) I would be very grateful for any information.