How To Count Characters In A Cell But Exclude specific letters?

Hi All,

I want to count the number of characters in a cell including spaces but, exclude specific characters. Is their a simple formula?

E.g.
Buy {Keyword:Baby Clothes}

I want to include spaces as part of the character count but excluding {Keyword:}. So in total, their should be 16 characters left that I can fill in a cell.

Reason being, I want my character count to starts from 30 in the cell. So my formula is =30-LEN(I3). I would like to add this is so that, when I add more characters it goes down. Also 30 is the character limit which is why I've added this and helps me know how many characters I have left.

Warm regards,
V