The golden rules: Measure once , if at all, just guestimate it, there or there abouts will do, cut twice, rebuild thrice, scrap the lot! It's what RU-vid thrives on!
Hello. I noticed something in your video which I wouldn't mind knowing where you sourced it from. On your Jigsaw can I ask where you got measuring attachment that allows you to cut straight?
It came with it when I bought it. The jigsaw a MacAllister 600W from B&Q and it has the piece with it as well as the mounting points for it fitted to it. Only goes to about 15cm though.
Use a circular saw with a guide to cut clean and straight....Screwfix sell the Evolution brand that is fairly cheap (about £50) but excellent value and has a brilliant blade that will also cut through old embedded nails hiding in the wood without a problem! You're welcome!