I’d go with 100% - it won’t be stronger than that. Otherwise you may find 75% doesn’t work and you need to print a 3rd one.
I’m not sure I’d use PLA in fact. If you can you might want to try ABS. That’s what I tend to use most with filament 3D printing. With resin I just don’t bother with infill - always print them 100% because the print time is the same and the difference in resin used is pennies. Not worth the time spent reprinting when something isn’t strong enough.