I've learned the hard way. I was increasing my seat to the point where the warning mark told me to stop, but I was still having issue with going over simple incline such as freeway overpass. It was then I realised the bike frame was too small for me. I've found a larger frame with the right fit allow my knee to be in a much bettter position relative to the crank and at the same time allowing proper height adjustment. After that, toe clip was another helpful method to maximize my effort.