I tried to come up with a general formula to calculate this for other crops as well, but it turns out to be pretty difficult. I find it hard to incorporate the fact that sometimes you're left over with 15g which means you cannot buy 1 parsnip, but in most investing formulas it will also calculate the compound interest over the 15g.

But over long enough cycles this does not matter that much. So you can approximate the ''real'' g/day using this formula:

Compound Interest = Starting money * (interest rate^growth cycles)-starting money

Starting money: it helps for accuracy if you pick an amount that leads to an integer if you divide it by the amount of crops you want to buy. (So don't pick 95 for parsnips, the formula thinks you get interest over the full 95 but in reality it's only 80 since you can't buy 4.75 parsnips)

Interest rate: sell price/buy price. For parsnip that's 35/20=1.75
Growth cycles: amount of growth cycles. In my example that's 3.

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Compound interest = 80*1.75^3 - 80 = 348.75 gold. In my calculation above it came out to 345, so it is abit of but it's still a way better estimate than the profit/growth duration formula.