I have absolutely zero experience with 3D printing and modeling, but for proportions...
A tried and true method for me is eye-balling. It's not precision accurate. However, I've seen lots of people (and done it myself) go through lots of calculations and end up with something that visually looks way off. I've done this on paper, thought my math was correct, but ended up with a part that was too big. Calculations are really important, but sometimes it's best to eye-ball it.
I use ratios.. compared to other parts that I know are accurate. If a bolt head is 1/4 the size of the lightsaber handle, and the handle is 20mm, the bolt head is probably around 5mm. Then I play around with 4, 5 or 6, to see if it looks better. Recently, trying to find the size of things on a Graflex based saber, I measured the details of the graflex with calipers, and compared those real numbers to the part I was trying to ID, seeing if it was bigger or smaller, and how much.
I've had very successful results, and have used it to eyeball when machining and working on items too. I mean, this isn't for work - I imagine machinists don't have this luxury