Does your side hustle income completely cover related expenses? You mentioned not having much profit from your side hustle which makes me think you’re breaking even currently, in which case I’d just leave the side hustle completely out of the calculations since its income covers its expenses. So for example, if your side hustle earns you $1000/month but the expenses are $1000/month (for a PROFIT of $0/month), and your living expenses is $2000/month, then your FIRE number is $600K ($24K x 25).
However, if you actually mean you don’t make much income from your side hustle, and its income is not sufficient to cover its related expenses, then I’d just figure out how much you’re spending on your side hustle expenses and add that to your annual expenses when calculating your FIRE number. So for example, if you normally spend $2000/month on your living expenses plus another $1000/month on your side hustle, for a total of $36K per year, then your FIRE number would be $900K (36K x 25).