A basic equity question

I am holding 10 9 hearts and now the flop comes

8h 7h 5c

When I put this in pokerstove my equity is:-

Against an over pair ( kings no hearts) :- 56% which is fine as I have 9 flush outers and 6 straight cards making it 15 outers and when I multiply 15*4=60% so roughly in that range

Now when I am against a set the equity reduces to :- 42% while now I lose only one outer (5h) and all the other outs should be live. so total is 14 outs so 14*4=56% the equity should be close to 50% why is it reducing so much.
I know my opponent has around 24% on flop to hit a full house by river. but how much that impacts my equity???

How to account this probability mathematically while playing a live game and we have no access to pokerstove.

Even when you make your straight or flush, the set will often have a redraw to a full house.

You can't, since you never know your opponent's exact cards. Using the rule of 2 and 4 is great for calculating the "chance of improving your hand", but doesn't tell you your actual winning chance, as villain may also improve his hand. With more experience you'll learn to think in terms of equity against a range rather than just your odds and outs.
By playing with Stove/Equilab, you'll find out that the equity of draws changes considerably when you give villain's made hands like sets, because of redraws. If you are hoping to hit a flush/straight, and put villain on a pocket pair, be very aware of the board pairing. Many of your "outs" are not true outs.
(If someone flops a set, then they have 7 outs to a boat/quads on the turn, and 10 outs to a boat/quads on river).