I know it's a small thing (literally) but has anyone successfully adjusted anything in order to increase the amount of finger gap you gain when pressing the button on the tailgate opener?

You really need to slip your fingers into a tiny gap to get right under it and lift.

Using the upper opening 'ledge' leads to either accidentally opening the top half too or leaves my wife with too little leverage to get the heavy boot lid fully lifted (she's only little).

Has anyone managed to increase the initial lift the solenoid provides?

If the main gate is open the upper glass can’t open. There are finger holds to either side of the center release for the upper glass, these are what my wife and I use to open the main tailgate. 2011 RRS HSE Luxury 3.0 TDV6 Fuji White on Ebony/Ivory
Born and raised in So Cal. Moved to the UK in 2018 for a few years. Now in Poland and looking at moving back to the UK.

The answer if to attach a pull strap to the bottom of the tailgate and leave it sticking out over the bumper . You can pull on that to get your first lift. Always listen to old people or when they die you will live on in ignorance.
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Learned to 4x4 on 100 mile beach and Frazer Island QLD
Dont spend money on old cars you will never get it back