Formula that creates a top up value where the total of another cell is sub a specific no

Hello everyone, new to this forum. Reasonable proficient in every day use of Excel for basic tasks and formula! Trying to expand my knowledge and automate some processes / invoices in our business. Help required please.

What I am trying to do is create a top up cell where the value of a cell is less than our minimum charge for something.

I initially did =500-F30
Which gave me the correct figure if I was trying to achieve 500 min where the value of F30 was sub 500 but if it's greater it becomes a negative figure. How do I make the calculation conditional or up until I achieve 500?

Any help would be appreciated!

Thanks, that worked perfectly!!!! Have a great weekend!