Attached is a photo to give you an idea of what I did last year. This is the best I've got because I did not take an "after" shot. Anyway:
Cut top where shown, re-edgebanded, mitred the fiddles back so it could open just past 90d. Hinged with flush two pin table hinges for flat surface. I used the less expensive hinges from ABI (purchased at Hamilton). Next time I would use a better grade of hinge because the Chinese made ABI's are inconsistent in size so the router template I made didn't fit each hinge as cleanly as I would have liked. They still look good, just took extra hand fitting.
The back portion of the top is 4-5" wide and fixed down with a block supporting the seam where it butts the fixed existing portion, otherwise methinks it would flex downward when weight applied to surface. I also routed a hole for, and fixed in, along narrow plastic tray in the newly fixed 4-5" wide portion. It holds parallel rules, dividers, pencils allowing large chart to extend over the top.
I stayed the top with a Sugatsune SS lid stay (McMaster Carr - #11615A11-Type 304 Stainless Steel Lid Support).
Works well.