Hmmm......seems pretty clear:
No person may qualify as a candidate for more than one public office, whether federal, state, district, county, or municipal, if the terms or any part thereof run concurrently with each other.
(4)(a) Any officer who qualifies for federal public office must resign from the office he or she presently holds if the terms, or any part thereof, run concurrently with each other.
(b) The resignation is irrevocable.
(c) The resignation must be submitted at least 10 days before the first day of qualifying for the office he or she intends to seek.
(d) The written resignation must be effective no later than the earlier of the following dates:
1. The date the officer would take office, if elected; or
2. The date the officer’s successor is required to take office.
(e)1. An elected district, county, or municipal officer shall submit his or her resignation to the officer before whom he or she qualified for the office he or she holds, with a copy to the Governor and the Department of State.
The real question is what the law means by “qualified”.
I don’t believe it means the familiar Constitutional eligibility standards (natural born citizen, age 35 or more, etc.)
To become a presidential candidate, you have to fill out a FEC form to that effect (the easy part), and take steps to get listed on the ballot in every state (the hard part). Each state has different requirements for that, generally requiring a certain number of petition signatures, paying fees, etc.
A state-by-state list is here:
https://ballotpedia.org/Ballot_access_for_presidential_candidates
I believe the law means that when a potential candidate holding a position in Florida begins this process by taking concrete steps in that direction (as opposed to merely talking about it), that is the point at which qualification takes place. When you form campaign committees, start raising money, circulating petitions, hiring staff, you are a candidate, and you need to file the resignation.