measure twice, cut once

I'm putting these door up in the kitchen. They're half glazed. The doorways in the house are narrower and taller than normal. When I fit them I will need to put something on top so they are the right height. I've bought matching doors. One doorway has a folding door alongisde. It's still glazed like the others. I borrowed my brother-in-laws electric saw. It's one of those battery powered circular saws. I need to take 1-1 1/2 cm off each side of the doors. The sliding door will be lower because of the runner it hangs from. I can't just hang lower because the glass won't be level so I will still need to cut some off and add another bit.