How long tent poles for flexible tarp setups?

Not enough information.

However, you can set up the net tent without the tarp and measure it, figuring that the poles would need to be a couple of inches longer to provide some clearance.

Modified pyramid depends completely upon the configuration. Pitch it using a stick and get your measurement from that.

Not enough information.

I know it’s quite scarce, but isn’t it quite standard with an 8 x 10 flat tarp when I have 8″ of wiggle room?

You need to set it up and measure. The taller the pole, the higher off the ground the edges will be… or the narrower the shelter if you move the sides closer to the ground.

I’ve never used a net, so I can’t speak to that.

Generally, the highest I’ll go in Pryamid mode with a flat tarp is 48 inches. You can always wrap tape around the pole to create a lower point for your guyline.

The carbon poles you are looking at (they’re what I use) aren’t cheap, so you want to be sure you get what you need the first time.

So the answer is…. measure.

You can buy inexpensive wood at your favorite big box home improvement store to experiment with at home.

Thomas, You can also make a tarp somewhat ‘self-adjusting’ by adding shock cord to the grommets on the tarp.   I made my tie-out loops  ~6″ long (about 15″ of cord each), and is what I use on a Gatewood Cape.

I know it’s quite scarce, but isn’t it quite standard with an 8 x 10 flat tarp when I have 8″ of wiggle room?

There is little that is standard with any tarp, especially basic flat ones. Preferences for overall height, side coverage for different conditions, bivy vs ground sheet vs net tent, physical size of the occupant, desired head room… all result in a huge range of possibilities.

Shaped tarps such as the Hexamid solo are pretty much limited to height variation. Simply offsetting the pole can lower the edges quite a bit.

My suggestion would be buy a cheap pair of trekking poles and use them as tarp poles for a couple of trips.  then you will know what you need.