A useful trick for shortening dipoles by small amounts is to fold the excess length back along the wire. Ie. you’d normally pass the end of the dipole through an insulator and wrap it around or tie a knot. Instead of permanently fixing the length, just wrap it around to make a resonance test, then calculate the adjustment needed, adjust the length by unwrapping, shortening by pulling more through the insulator, then rewrapping. By wrapping I mean simply winding the free wire around the other wire several times.
You can fix the length once adjusted using any number of methods but leaving it twisted should last 10 years. My 80m dipole is an adjustable dipole using this method. I first used it about 40 years ago, it went to Brunei with me in 88 and had to be lengthened a bit due to the coconut trees at each end being a good height, but back here the ends are closer to the ground so it needed shortening again. No wire was cut during this time.
There is a neat but much heavier method used by the Rak (or Raic?) antennas I bought many years ago. They have parallel overlapped sections of uninsulated wire with two shorting bars, allowing the length of the antenna to be adjusted within the length of the overlap. Too heavy for a portable dipole.
Anyway, hopefully the combination of the calculator method (online or via the basic principles outlined by Andy) and a non-permanent termination method will make the process quicker and easier.