Label the remnant while the first project is out
After cutting for one quilt, measure each useful leftover rectangle and put a label with the fabric. Include the usable width and length, fabric name, print direction if it matters, and the project it came from. If you're still planning to use part of it in that quilt, note that reservation before putting it back on the shelf.
Keep the cut list with the project
Export or print the first quilt's cut list and mark which pieces you've actually cut. The diagram's unused space isn't a measured remnant: trimming, squaring and cutting choices can change what remains. Check the physical fabric before offering it to a second project, especially when the new pieces only just fit.
Give the next quilt its own count
Work out the second project's cut sizes and quantities separately. Use only fabric that isn't still reserved for the first quilt. The square cutting calculator can check a rectangular remnant, and the strip set calculator can check measured strips. Neither tool keeps a shared stock count or subtracts fabric when another project uses it.
Use names you can recognize later
Save plans with names such as “Blue sampler — backing” or “Garden quilt — binding.” My projects keeps the plans in this browser, so export the project files if you need a backup or want to use another device. Keep allocation notes with the fabric itself; a saved calculation isn't a live inventory record.
Measure again after the next cut
Once you cut for the second quilt, replace the old label with the remaining usable dimensions. Cross out the allocation on the project list as well. That small update is what keeps a tidy-looking digital plan from promising fabric that has already become part of another quilt.