It's impossible. They are round

While I like to have a base line for slope - each of my games are set differently. While I personally like fast play there are a couple of my pins that are not quite as steep as others. It's good for friends and family that are not regular pinball players.
--
Jeremy Agema
Central WI

Took me a few seconds....lol

Screenshot from the app "Laser level". Works will enough for me. I took this when I was setting up my Firepower. It has more pitch now, 4.7 was slooooooow. I had to buy longer bolts for the back legs.

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I used to go all crazy with an incline level, I have a very nice Starrett machinist level set, way more precise than necessary.

I came to realize virtually all of my machines if I put the front legs all the way up and 3" long levelers on the back all the way down with a nut on the top and bottom side of the back leveler (because I am a belt and suspenders kind of guy). That would give me ~5.5 to 6.25 degrees on all of my machines.

Incline was the hard to part to get "right". I just don't worry about it now. I do the above adjustment and then level the front and back for level by putting a small level on the playfield as far forward and backward on the playfield as possible.The smart phone app is good enough for incline if you want to get a rough idea of what it is.

The level left-right is what really matters. If that is off it will drive you crazy. I use a good, small spirit, not electronic level that is twice my age. I would recommend the same. Get a small, precision machinist level that will measure to a level of precision way beyond what makes sense for on a pinball game. Look around on ebay and other places where estates are getting sold off, they often sell for a song. They are useful for many things in life, never need batteries and if you keep them from rusting they will still be just as good when your grand kids use it 100 years from now.