If you have the option of using SPY, I suggest it. But you may not have that ability. If not, any S&P 500 index fund will do. You’d just note when $SPX hits those levels and when it does, do a buy into your index fund (whichever one you choose to use). Yep, each index fund will have different per-share pricing. That doesn’t matter. Remember, you’re doing equal dollar amounts per tranche. So you’re putting in the same amount of dollars in each tranche, no matter what index price per share your fund is the equivalent of $SPX.
The supports are based off of previous retracement percentages along with long-term support zones on weekly, 10-year charts.