Easiest way to create Large Vertical Totals is based on an existing working mode.
1. Use ToastyX
2. Load the working 240 Hz mode
3. Put radio control on "Vertical Total" and "Pixel Clock"
4. Manually type a new number in Vertical Total until you see target Hz (never edit Hz directly manually, you have to edit Vertical Total to create the new Hz. Increase VT to decrease Hz).
You can use NVIDIA Control Panel by multiplying the original refresh rate vertical total by (OrigHz/TargetHz). So multiply by 4 if you're going from 240Hz to 60Hz with all the other numbers in CRU untouched (except Vertical Total and Hz). Pixel Clock and Horizontal Scanrate remains exactly the same as original working max-Hz resolution.
If you wish, you can first create the most reduced-blanking possible max-Hz mode, before deriving from it, but I just use create my custom QFT modes from the standard 240Hz EDID.
Update: I made a new thread:
HOWTO: Quick Frame Transport (Large Vertical Totals)
Eventually I'll create a Large Vertical Total Wizard to make things easier, at least until manufacturers build in a QFT (Quick Frame Transport) option into monitor menus to activate Plug-n-Play QFT EDIDs on a single button press. That could make CRU tweaking obsolete for QFT someday.