I assume these are flat w.o. The system bindings. If so, measure the two contacts points where the camber touches the snow, decide by 2, that is your ball of foot, your boot center will be about 2-4" behind that.
Philplugs method seems a bit impercise. The traditional way to mount K2 race skis was to devide the stated cm length by half, in this case 181cm and measure half the distance from the tail and you have your toe point. It doesnot matter what the actual ski length is just what the stated factory length is. In this case it would be 90.5 cm from the tail. A long turned up tail would throw this measurement off, of course. Take this with a grain of salt but this method should be close IMHO.