Well really you should not have
truncatememory there at all. When the value is completely removed from the BCD store you should have access to all of your systems memory. You can either uncheck the Max Memory, click OK then check the box "make all boot settings permanent", click apply and ok.
OR
From an elevated command prompt you can type
- bcdedit /enum if you only have the two entries {bootmgr} and {current} then you can type
- bcdedit /deletevalue {current} truncatememory
- If you have more entries you would replace {current} with the {really-long-GUID}