Not a freight f/o so I can't tell you what the rosters are like, what I can tell you however, is that in recent history (18 months), and in particular the last 4-6 ish months, rosters have been arguably the best they have been in the last 5 years.
Sadly the cynic in me must add that I think this is the result of "little Nicky's" bold new position, and attempt to convince us of what a great life we have. (glossy newsletters with pretty graphs of days off etc) So why would we possibly want to take them to court to go back to the old rostering system?
Days off before and after leave, attempts to adhere to the old 543 rule, access to the master roster, both on the a/c and off, it seems just like the good old days. The downside is, they won't stick to it. Sadly we all remember the punitive rostering of recent history, so to answer the question of the initial post, perhaps you would be better off asking for a copy or this weeks rostering practices document "which may be amended at the companies discretion from time to time"
If they can't/won't give it to you, but can show you the April, or May roster of a freighter f/o, you may want to consider long term what it may be like if there were no war, or global economic slowdown.
The new 900 hours per year doesn't sound too bad really. Hell 42 days leave (or is it less on the freighter?), it's only 85 hours per month. Position you from New York to Anchorage, or worse Toronto to HKG, you only get 50% credit for duty time. On reserve? 1 in 5 credit! There is no shortage of gotcha's.
Caveat Emptor