If the ones you have 'plonked in' previously have done OK then your plonk is about right! We plant at about three inches deep but I certainly don't measure it.
[edit] I have just been looking at one of my bulb suppliers catalogue and they advise six inches deep. I have never planted tulips at this depth.

Have just received some bulbs from Walkers and the booklet says to plant them twice as deep as the bulb is tall ( so a 2 inch tall bulb gets 4 inches of soil above it ) and leave 2 bulb widths between each bulb.

Rule of thumb with everything you plant or sow is 4 times the size of seeds or bulbs. I have put bulbs in upside down when mass planting for natural long term displays and they all come up and give good results for many years.

Ours started off at these sort of depths but many of the Tulip Attila are now just below the surface and still flowering (hopefully, they did last year).

Plenty of depths there to choose from, Snoopdog ;-))