Ah Barit you've been in one of my landings.
If you take 1"g" as 32 ft/sec/sec then all you need to know is the time it takes the undercarriage to stop the vertical movement ( this ignores wing flex etc which may be quite considerable.
So if we assume the time taken is 2 sec then 1 g would be (32*60)/ 2
or 960 ft/min rate of descent.
however, N2 over oil oleo's do not give constant decelaration so i do not think there would be an easy conversion