How to cut soap consistently well?

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How do you manage to consistently cut straight, parallel lines using a guillotine to make bars correctly reflect your label's stated weight?

After figuring that weight loss of bars for one of my recipes is 4%, I aim to cut bars at 104% to 105% of the label weight. Do you do this? Using the guillotine below (with the red arrow in it), the results have been disastrous. For instance, when cutting, some bars are either way too high like 150% or just below at around 97% - 98%. What do you do with your imperfect 150% and 97% soaps? It seems messy to price each bar on its unique weight. Also, the non parallel nature of the lines in some instances is so obvious that won't it simply look too unprofessional? Are moderately non parallel soaps acceptable to customers ... or just seem too unprofessional (even for a newbie seller)?

Will a new guillotine with an adjustable piece (as pictured in the image with a circle in it), resolve my problems with non parallel lines? ... and or do I need to improve my technique of properly anchoring the soap as I cut it?

I could ask a wood worker in the area to build a new guillotine. My log molds produce bars with dimensions of 3.25 by 2.25. Would the dimensions of the last image be good for resolving my problems? (Of course the wood worker will include the adjustable piece despite the fact that this 3rd image if my current guillotine does not show it). Happy to see your designs or design ideas.

one other nuisance is that the knife sometimes gets stuck as opposed to sliding all the way to the bottom. In such cases, it is cutting into the wood of the inside of the slot. Should the slot be thinner?

Also was wondering if it would be a good idea to also make one of the long sides adjustable ... or whether that would be an overkill without any benefit. If anyone wondered why I thought of having 3.5" tall walls; it is because I wanted the option of cutting with the soap oriented the portrait way. Currently, the walls are too short to accommodate that.

BTW, regarding wire cutters. Is it ever possible to adjust strings on the wire cutters to different widths, ie if one needs to cut specific weights?
I bought a lovely loaf cutter from Bud’s Woodshop in Etsy. It is a wire cutter and gives me uniform bars every time - I can do a loaf at a time. I would not trade it for anything after using the type you show and after trying to “free hand it.”
My husband built a cutting box for me : basically an open ended box w a guitar string (wire) strung across the opening at approx 1", which is the thickness I wanted my bars to be. He drilled a tiny hole on each side of the box to thread and secure the wire through; both holes need to be at the exact same height. Anyway, I place my loaf on end at the opening of the box and just in front of the wire then steadily push the loaf through, slicing a very precise bar of soap. I have another wire at the opposite end at the correct height for cutting my bars in half for a more manageable sized bar. The cutter is awesome plus cheaply made from scrap wood (and to whatever dimensions you choose); my bars are also always consistent in size which is so gratifying!
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Practice many years. Deep breath first. Clear mind. Cut straight up n down using shoulders and core not arms n back. W my fudge cutter I originally learned with I accidentally cut 68 bars for BOTH batches on my xmas soap this year. That's consistent esp bc I just cut em like I like em n be sure to get at least 15 bars per 5 lb loaf but when weighing and counting last 2 batches surprised to have gotten exactly 68 bars per batch.
Also surprised to see cure drop water weight from 20.2lbs immediately before cutting to 18.2 lbs barely three weeks later. Wow. I actually had no idea. I don't usually measure that. That's a lot of water to evaporate and just shows how the bar keeps silently & mysteriously curing at room temp unbeknownst to the soaper the universe is still working its forces on those cut bars of soap that are droppin' weight like a wrestler desperate to pass his weigh-in for a match.
just noticed after somebody liked my comment that I think I made this reply before properly understanding the question. I thought you were talking about a pastry knife. My answer wasn't very responsive, was it? Sorry 'bout that. I actually don't even know what a guillotine is outside of a discussion of the French Revolution. You guys are all a little more advanced than myself, i think.
Image of miter box cutter below is what the hardware store guy sent me via WhatsApp re miter box cutters. However, he does not know much about the cutter. Would be happy for your thoughts to save me a possibly wasted trip to the store.

Can someone else please tell me whether the hole, projection at the bottom of the frame or something else usually has some type of adjustable stop mechanism that will allow me to make bars of roughly 1-inch thick? I see one set of slots just above the product label. Should I imagine that there is a second one hidden below the label? If not, is there some way this will allow me to make 1-inch thick bars? .... or should I just ask the woodworker to custom make a box cutter instead?
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Looks much like the one I use, only mine has inch marks. Bought mine from Home Depot about 20 years ago. You'd think it would have the marks since it's a construction tool. 🤔