I no longer have any of the mulitfloral paph species in the collection. But what I do remember doing with them was growing them underpotted. For years I was unable to flower the big species; rothschildianum, stonei, etc., until I downsized the pots they were growing in. I noticed that these species, including sanderianum, liked to dry out somewhat between watering. They did not like to remain moist all the time, which is akin to their epiphytic nature. So the smaller pots allowed very rapid drainage and frequent watering to occur. I also grew them in very bright light, as one would use for growing cattleyas.

At the time I was growing them the greenhouse was well insulated and heated. I would receive a natural drop in temps at night, but I wouldn't allow it to get past 55 deg. F. It would have been better to keep it at 60 deg. but the plants didn't seem to mind. I also fertilized with 20-20-20 year round with every watering, diluted to 1/4 strength. But during high summer I would increase the strength to 1/2 stength (1/2 tsp p/gal) every other watering, allowing a flush through of pure clean water. I also repotted all of the paphs on an annual basis. Paphiopedilum do not mind being repotted so often.

I don't know if I would invest too much worrying in night time temp drops. I have heard that some of the best multifloral paph growers live in Florida and grow their plants in open end greenhouses. Only excellent culture and time will reveal if your efforts are the correct recipe. no attention to the old cliche' that multiflorals require a minimum of 10 years before they flower. That's not true at all. Many, sanderianum included, can grow from seed to bloom in 6 or 7 years, albeit under the best growing conditions.

Good luck with your Prince Edward of York. I had several of those and managed to flower one. They're enormous flowers on really tall spikes. Hybrid vigor to the highest degree.