Three words: peel, stick, repeat
When shimming a small amount evenly across a wide opening conventional shims are difficult to get level and don't provide adequate support for door sills. By using layers of a dense membrane—a heavy peel and stick membrane in this case—it is fast and easy to build up out of level surfaces and provide solid support.
With a level (and straightedge if necessary) determine how much shimming is required to make a level surface. In this case the subfloor was flat but about 1/4" out of level from one side to the other. Sometimes the surface to be shimmed is sloped AND dipped or wavy.
This method works well in those situations too so just determine how much shimming is needed, and where, to plan out shimming layers
Since the floor was evenly sloped over the 55 inches width, I made spacers of 1/16 inch thick material in 11 inches strips to get the 1/4 inch build-up needed.
Cut even strips and place them across the opening. #30 roofing felt is another good choice for stacked shim layers.
Once shims are in place and secure, proceed with door installation normally using membranes, sill pans, flashings and sealants as required.
You can be confident that the built up shims will provide all the support needed for the life of the door installation.
—Matt Jackson is a master carpenter, remodeler, SketchUp Wiz, YouTuber, and contributing editor to ProTradeCraft. He lives and works in Rapid City, South Dakota.