I've used varifocus lenses for 30 odd years and manage reasonably well.I think the advantage I have is we got used to parallax prioblems in the 80's at Markyate and to avoid it we learned the same head position was enabled to avoid it.This probably carried on till today.
Ensuring the sight picture has an even dark ring surrounding it soon shows potential errors if the evenness changes.
Fighter against the 'Dark Arts'
Dark keeps moving in on the day.
Steadily going downhill faster!