Yes, that's right. You don't want to go over 20gal for a baby and even then you'd need to clutter it up a fair bit to help it feel secure. A 40gal is too big for a hatchling even with extra hides and plants. An insecure baby will eventually go off feed. A tub is a good temporary choice or an older juvenile or young adult might be a better choice. You should still be able to pick up a young adult pastel, spider, pin, or even perhaps a lesser for under/around $300.
And also, your probe for the t-stat should be under the substrate directly on the glass over the flexwatt - not on top of the substrate.![]()