We use knifeless all the time with reflective, specifically 3m 580 / 780. Works great!
Sounds like op just wants to print a decal such as "Jo Jo's handyman services, blah blah blah" and make it fit inside of the indent so you're not going over the hump?
In that case a online template from pvo is accurate enough. You may want to give yourself half inch wiggle room top and bottom to make sure you're not going over the hump.... Plus you don't wa t to end right on the edge, it's better to leave some space so the text can be read.
If you're after a full coverage decal... As mentioned buy a roll of knifeless tape, add an inch bleed on each side and just knifeless it. It'll be much more cleaner and accurate than pre cutting the full coverage to size.
(Edit) also, if you've had issues with reflectives and knifeless tape... There's a special version of knifeless.meant for the thicker materials. Trying to use bridge line or something might not work the best... But their precision cut works great.
https://www.3m.com/3M/en_US/p/d/b00033800/