OK, here is a comparison of 3 different RFM96 modules, with the 1/4 wave vertical element being trimmed.
Note that the 3 modules have different responses and slightly different peak signal lengths.
The variation was actually less than I had expected.
Somewhere around 7.9cm seems a good compromise, around 91.4% of the actual 1/4 wave length at 868mhz, elements were 1.5mm diameter copper wire.