Notes and tips.

Although a lower SWR is usually better, most antennas will not tune down to 1:1.
You should be able to get the SWR down to 1.5:1 or better on your middle channel, although less than 2:1 should be OK. The SWR at the top and bottom channels will be higher than in the middle.

What do you do if you have cut off too much, and cannot now tune the antenna? Strictly speaking, the antenna is now, to use a technical term, knackered.
If the antenna is the type with a steel whip in the top, you may be able to buy a replacement whip.
With other types, there may be a cheap and dirty trick. Some people have been known to use electrical connecting blocks to add pieces, but this is messy.
Antenna lights and capacity hats which connect to the tip become part of that antenna, so adding one of these may reduce the required length of the main section.