Former restaurant cook here. 26 inches may be the suggested depth from the wall. For example, the general height at the bottom of the hood for a restaurant is 75" / 6' 3" from the floor.

If your cooktop is about 35" or so in height, 26" would bring it to just over 5' from the floor. It would be uncomfortable to work with for a short person never mind a taller person. It would also be a fire hazard if this were an open flame cooktop.

Congrats on your new hood!

Cathy

The higher the hood the less effective it is, but I believe most recommend a height that is less than 36" above the cooktop. I wanted 30" but my cabinet guy mis-measured and I ended up with 32". It's okay, but not ideal as there is reduced capture. My last kitchen had the hood at 25" above the cooktop and that was too low.

Just for general info....when I'm planning a kitchen and have an upper above the range/cook top, my local building code states a distance of 30" (36" if gas) above the cooking surface to combustible (the upper cabinet). Your typical hood would come down from that and the hood would be around 4" in height. Which would come close to the 26" you've noticed in the installation guide. That's not to say you can't put it higher though. Do keep in mind that the higher you go, you would require a hood with greater CFM since it has to suck from a greater height. Commercial hoods are much greater CFM so they can be that much higher.

What's the hood make and model number?

Most hood manufacturers recommend mounting the hood 30" above the cooking surface.

If you install it higher, then you will need to increase both the power and the size of the hood. Why? Because as smoke, grease, steam, etc., rise, they expand as well as rise. The higher the hood, the more the hood has to work to capture and exhaust the smoke/grease/steam/etc. - so you need to increase the capture area to accommodate the expansion as well as a stronger fan to catch and expel.

Our hood is installed 30" above the cooking surface (a flat glass surface) and it's not an issue for our tall family. We range in height from 5'10" to 6'7". Our hood is 42" wide x 24" deep over a 36" cooktop. The only time my 6'5" DH hit his head was when he stuck his head inside the hood and then backed away b/f getting out of the hood! He hit the back of his head!