If using frozen rhubarb... measure frozen, then thaw. Drain in colander-don't press juice out
Processing time is for 1,000 feet of altitude and lower. Be sure to add a minute of processing time for each 1,000 feet higher in altitude you are.
"Sheeting process": Dip a very cold metal spoon into hot marmalade. Lift up spoon; hold it horizontally. The syrup will run off the edge of spoon. As it cooks, the syrup will usually run off in drops, two at a time. Keep cooking. Once jam has cooked enough that the drops mix together and "sheet" (blob together in solid sheet) off a very cold spoon, you've reached the gel stage.
Rhubarb-Orange Marmalade https://www.thegratefulgirlcooks.com/rhubarb-orange-marmalade/