Different schools will have a different % to 4.0 GPA scale so this is something you can ask your counselor. 88% would be a B+ or A-.
Googling an 88% brings up a wide range of conversion scales but if you plan to convert, then you have to convert each grade not just the overall GPA.
Unfortunately, the UC’s do not consider Senior year grades in their application review but maintaining good grades is important to keep your acceptances since they are provisional