what to expect from rustys 3.5" add a leaf

my brother in law just gave me a pair of 3.5" rustys add a leafs. i wanna put em in but i was wondring if they give a true 3.5" or more or less. i expect them to sag a little over time but i figure theyre either gonna start too high and sag to about 3.5 or start at 3.5 and sag to 2.....anybody running these? and whats your experience with them? thank-e!!

firends 91 XJ started at 4" then COLAPSED to 2" after 15,000 highway miles...


BUT he didnt put the spring clips back on - and they all faned out...