The datasheets have quite a large spread between the typical and maximum leakage specs. This is because testing to low leakage takes time and is expensive.
The numbers, especiall for the test limits are different between manufactures, so the number can vary somewhat From the DS I have, I get
Philips BAV199: 3 pA typical and 5 nA max at 75 V (rather high voltage)
Interfest 2N4391 100 pA max at 20 V
Fairchild An6609 process 51: typical current near 15 pA for 20 V
The BAV199 has a low typical leakage, but a rather crude test. The main difference for the way more expensive PAD1-5 is that they are actually tested to lower currents.
So chances are that at some 10 V the leakage of the PAD , BAV199 and 2N4117 JFET gate is similar, just the price is different (some $20, $0.20, $5 ), refelcting the test limits of some 2-5 pA , 5 nA and 10 pA. I would be surprized if the lower cost MMBF4117 are actually tested to 10 pA - more like they accidentially coppied the data from the 2N4117.
Diode leakage may be an interresting thing to measure with the DIY nV meter. Just make sure to keep RF out.
Edit:
For testing the TIA type meter, the offset / bias current is measurend with an open input. The TIA does not like the input shorted and than gives erratic errors from the OPs offset voltage. So the test case is an open input. The capacitor in Fb should be relatively small, maybe some 10 pF (preferrably PS type) or the like.