Learn English – the proper way to mention “current” for future events

In theory, this should be easy. Drop "deceptively" and you have the essential quality of the subject:

"The floor is deceptively flat" → The floor is flat.
"He's deceptively strong" → He is strong.
"He looks deceptively strong" → He looks strong.

Now add "deceptively" back in to indicate how the observer has been deceived:

"The floor is deceptively flat" → The floor is flat (but appears otherwise).
"He's deceptively strong" → He is strong (but appears otherwise).
"He looks deceptively strong" → He looks strong (but is otherwise).

The difference between #2 and #3 is the difference between "looks" and "is." This has the effect of flipping the comparison around: in #2, you are deceived about how he is, and in #3 you are deceived about how he looks.

In theory, it's easy. In practice, you're just as likely to find the word used to exactly the opposite effect, so in most cases you should assume you're going to need to get the meaning from context. I'd recommend staying away from "deceptively" entirely when writing: "The floor is flatter than it looks."

I think the difference between the two types of examples that you've exhibited is the relative placement in time of the action in the "if" clause, and the action in the other clause.

• If it rains in the evening, we won't go for a walk - here, the event of raining occurs BEFORE the decision about whether to go for a walk.
• My teeth will rot if I eat too much sugar - presumably, I'll be eating the sugar BEFORE my teeth rot.
• If it will significantly increase complexity, don't implement this feature - here, the increasing of complexity occurs AFTER the implementation of the feature.
• I will give you money if it will make you happier - here, you becoming happier occurs AFTER I give you the money.

In all the cases where the "if" part happens first chronologically, we use the present tense. In the cases where the "if" part happens second, we use the future tense. However, because sentences of the first type are far more common than sentences in the second type, a good rule for learners to adopt is "don't use the future tense with IF".