Right way to use "Cannot but help"

A relatively straightforward use of "cannot but help" is along the lines of

When you use ... you cannot but help notice...

No issues there. However, I would like to say

If you have used ... (right form of) you cannot but help notice

I could simply duck the issue and write

If you have used ... you will have noticed

but not being able to work in cannot but help is bothering me. I'd be most grateful to anyone who might be able to tell me how it could be used with "If you have..."

I think you are mistaken. The correct form of the expression is "cannot help but".

A correct sentence would be: "If you have used the soap, you cannot have helped but notice the odor it has."

There is no correct way to use "cannot help but." There are two traditional and synonymous idioms, "cannot but [do something]" and "cannot help [doing something]." Those who say "cannot help but" are conflating the two (in much the same some people conflate "irrespective" and "regardless" to create the nonsensical "irregardless").

In an earlier posting of the same question (now closed), someone very helpfully gave a link to this Google Ngram, which helpfully demonstrates that while the use of "cannot but" and "cannot help" has become steadily less frequent since the 18th century, while "cannot help but" has grown slowly since the start of the 20th, use of each of the first two still comfortably outstrips the third.

That said, "cannot help but" has become common enough that many people won't even notice, but those who do may dismiss the speaker as poorly educated and/or not focused on the meaning of his words.

As for correct usage, either "I cannot but deplore ..." or "I cannot help deploring ... careless use of language" will suffice.