Figuring out non-obvious suit-handlings at the table?

After a bidding, where opponents remained silent, I recently had this trump-suit combination at MP-play:

K974

J865

The contract was cold, and I just wanted to take the best percentage to maximize number of tricks.

My "intuition" would tell me there is no better chance than to simply lead spades to the King, and then, if it loses, spades towards the Jack, and I may then have a 50/50 guess whether to finess or put in the Jack.

But NO. My intuition sucks here!

When I checked with the SuitPlay app, it tells me that the best play ALWAYS (regardless of whether you need 2 tricks, need 3 tricks, or just maximize the tricks at MP) is to start with small from South and (if LHO plays low) play low from dummy.

Hmmm..... now, why is that?

Short of just counting all the possible opps combinations and summing up the percentages for each one, I have no idea of how to approach this problem at the table. Counting is what SuitPlay does for you, of course, but we are not allowed such tools at the table.

So my Question is:

Are there any generally useful methods or logic, etc, one can apply to non-obvious suit-handlings -- not just this specific one -- but all the countless other combinations, too?

Or is there some basic flaw in my "intuition" I can somehow correct/clarify/"polish", and get smarter at this?

Learning all the possible suit-combinations you may ever encounter by rote seems like a quite daunting task, to say the least.