When your credit history is short (very little in it), it can jump up and down easily.
Credit score is a combination of many things. Longevity, length of time of account in good standing, moves score up.
Number of active account types gives a higher score.
A credit score is an instant in time. Your percentage of credit utilization is a short term movement.
Paying down the debt, reduces credit percent use, giving a higher score.
Credit Karma may use Vantage score on a different scale as FICO, the more common score.
Be aware that paying interest has no effect on score.
If you use a credit card and do it frequently instead of debit or cash, and pay statement balance every month, your score rises for longevity.
No way to know the precise number it will go to.
I'm surprised you are holding 618 with a maxed out card.
If you get a 726, it isn't a real FICO score.