I have a 4145B in my rack too--it's going to get bumped in favour of my shiny new-to-me 89441A pretty soon, and banished to the auxiliary lab. The 4145B actually has a usable lowish-femtoamps range.
Thanks again!
Phil Hobbs
(Typing to the melodious sound of my nice new Cheap'N'Cheerful Chinese (TM) 4 kW generator that got wired into my house today, just ahead of the snowstorm. Four bills for the gen, fourteen for the electrician, but it's been five days since we had heat. I'll probably get sick of the sound, but not today.)