How do I set Trailing Buy order by value NOT by token quantity?

I understand how the trailing purchase works however the quantity of coins I can purchase is all the time capped by the restrict worth.

If, for instance, ETH is $500 every and I have $500 to spend.

If I set my restrict to 500, I'll place an order and get 1 ETH. Nevertheless, if the worth retains dropping right down to 100, then I'll nonetheless solely get 1 ETH because the limit area is in quantity not value, which means I'd miss out on shopping for four ETH and end up with 1 ETH and $400 doing nothing.

What am I lacking?