Cutting acrylic bath panel to fit profile of baseboard trim
I'm guessing the "panels" will cover the existing (fiberglass?) tub walls? The skirting (molding?) from your photo looks to be made of MDF (medium density fiberboard). MDF won't last very long in a wet environment.   To install the acrylic sheets, if possible, remove the skirting and then measure the wall height and subtract 1/2-3/4 inch. The panels will easily install and the skirting will cover the gap.