I'm working on a GUIDE for this. Here is the portion relevant to the DVMT modification.
Prepare for build starting in Windows:
Place my ARCHIVE folder in the ROOT of a FAT32 formatted USB STICK/DRIVE for booting later.
INSTALL THE LATEST BIOS - mine is currently A03 from June 15, 2015. I already had what I thought was A03, I was incorrect.
If you had EXTRACTED YOUR DSDT BEFORE UPDATE, you MUST extract it again!
Make space for your install(s) by shrinking the Windows 8 partition.
1) Turn off hibernate in Windows 8 -
http://www.sevenforums.com/tutorials/819-hibernate-enable-disable.html
a) 'Search' for CMD (picture in ARCHIVE folder, SEARCH.jpg)
b) Right button click - Run as administrator
c) Run command: powercfg /h off
2) Make room for the Mac (and Ubuntu, if desired)
a) Shrink the Windows drive to the size you need; mine is 60GB as I do not really use windows for much
b) You need to establish the rest of your partitions now, under Windows, as the Windows GUID scheme isn't totally compatible and the MAC OS will NOT modify any partition in either size or add/remove - you do not need to format them; just allocate them.
- I used 20 GB for Ubuntu and the rest for the Mac drive(s), approx. 830GB. Mine is divided into two drives, 90GB and 730GB but that is not necessary nor would I necessarily recommend it. My previous hackintosh had 2 physical drives so I stored my users data on the second drive. Don't do that unless you are familiar with the process.
- I left the DELL recovery partition in place - I have actually restored the Windows parition with the recovery parition multiple times since my Mac build ... I dislike Windows.
3) Modify the DVMT video settings
a) Put my ARCHIVE folder on your windows drive. It contains the three programs you'll need to obtain your SETUP_VAR offset as well as pictures of the process referenced below.
b) Download the latest BIOS from DELL and install it; mine is A03 dated 2015-Jun-5. Reboot.
c) Execute 'AFUWINGUI.EXE' from the ARCHIVE - we have AMI AptioV UEFI on our machines (AFUWINGUI_1.jpg)
d) Press SAVE; I named my file DELLA03BIOS.ROM - it will display a window exactly like that you saw when you updated your BIOS.
e) Press EXIT (AFUWINGUI_2.jpg).
f) Execute 'UEFITOOL.exe'.
i. OPEN your BIOS ROM file saved in step 3.d above.
ii. Follow the two pictures: UEFITOOL_2.jpg for part one and UEFITOOL_3.jpg to locate 'SETUP'.
iii. Point to PE32 image section and using the right-button click, choose EXTRACT BODY and name the file to be saved; I called mine 'SETUP.BIN'.
g) Execute 'Universal IFR Extractor.exe'.
i. OPEN your SETUP file saved in step 3.f.iii above (U_IFR_E_1.jpg).
ii. Search for 'DVMT' (U_IFR_E_2.jpg).
iii. Note your offset and the value representing 96MB; you will use them in the next step (U_IFR_E_3.jpg).
h) Exit Windows and reboot. Press F2 to enter setup.
i. Right arrow to BOOT
ii. Disable SECURE BOOT; this will remain disabled from now on.
iii. Select 'Add Boot Option'; choose your USB key or disk containing the ARCHIVE folder.
iv. Using the MENU presented, choose successive folders, proceeding thru the 'EFI BOOT' folder, until you've selected the BOOTx64.efi file - enter a name such as 'EFI BOOT' and save the entry.
v. EXIT and SAVE setup; upon REBOOT, press F12
vi. Choose EFI BOOT
vii. Key 'setup_var 0xaaa 0xv' (wthout the quotes) where 'aaa' is the offset you recorded in step 3.g.iii above and 'v' is the value you recorded in that step; press <Enter>. You may receive a warning message; just ignore it. AS AN EXAMPLE ONLY mine would be 'setup_var 0x1c3 0x3' (without the quotes).
iix. Key 'exit' (without the quotes); press <Enter>. Reboot.
ix. Double-check the Advanced video settings to insure a DVMT of 64
THE WINDOWS PORTION OF THE INSTALLATION IS COMPLETE!