Basement – Sizing Electric Baseboard Heat (Supplemental)

I have a 635 square foot finished basement insulated with closed-cell spray foam on the concrete. The climate here is zone 5A (Central NJ).

I want to add supplemental heating via electric baseboards and I'm not sure how many feet of them I need.

I read that a typical rule of thumb is to take the square feet * 10 to get the number of watts you need and then divide by 250 watts/foot to get the number of feet. (So talking 25.4 feet here).

But I believe this calculation is if you are only heating with electric baseboards. I'm looking to use these to supplement my forced air system.
I have extended the forced air heating/cooling system into the basement by adding two registers and two returns, but I have only one zone for the whole house.

In the winter, when the forced air heat is running, the basement is 69/70 degrees, which is perfectly fine. When it shuts off for an hour or so, the basement drops to like 65/66. During the summer, the basement is 65/66 consistently (this is after I insulated the a/c ducts in the ceiling).

So I only really need to bump things up a few extra degrees to keep my kids from complaining.

(I would consider zoning the house into 2-3 zones. I think that would fix the basement issue in the winter. But during the summer, since I couldn't run heat and a/c at the same time, I'd be in the same position where its 3-4 degrees too cold.)