I have read all over the internet that level is more important that flat, i am doing a hall that is 40 X 100. the hardwood is being installed right-left which is flat but their is a difference in height from where i am starting to install the hard wood to where its going to end hence, a slop. the difference in height from the beginning and the end is about 7/8.

not sure weather i should take out the sub-floor where its high and plane the joist in that area.

Other way around....flat is more important than level.

Sorry meant Falt more important than Level. my floor is Flat but not level. the 100inch across is flat but the 40 inch across if not level and there is a difference of 7/8

Sand any bumps or use an electric plane, then build up the low spots.