Pricing a gang sheet by the square inch
Most gang sheet pricing arguments end at the same place: the film is a roll of fixed width, so what you sell is length, and length times width is area. Price by area and the rest follows. This is how we work out ours and the mistakes we made first.
Start from a sheet you actually printed
Take a real sheet, 22 by 60 inches say, and add up what it cost: film by the foot, ink by the millilitre (your RIP software logs it), powder, the printer's share of its own cost per hour, the press time, and the minutes a person spent on it. Divide by 1,320 square inches. That number, before any margin, is the floor. Shops are surprised by how much of it is labour and how little is ink.
Then decide the margin. A rate between two and three cents per square inch is common for standard DTF at the time of writing; what matters is that yours is built from your costs, not copied from a competitor whose costs you do not know.
The minimum charge
A 12 inch sheet at two cents is $5.28, which does not cover loading the film. Set a minimum charge that pays for the handling of any order, and a minimum length that matches the shortest piece you are willing to cut. The two are different settings: the minimum charge is money, the minimum length is film. Both should be visible to the customer before they build, not discovered at the cart.
Rounding
Nobody cuts film to 37.4 inches. Round the sheet up to a step, one inch for most shops, and price the rounded length. Say so on the page. A customer who sees their 37.4 inch layout billed as 38 inches with a one line explanation accepts it; one who sees an unexplained number does not.
Volume breaks
A longer sheet costs you less per inch: one load, one set of margins, one invoice. Pass some of that on with a lower rate from a length you choose, 100 inches say. It changes behaviour: customers who would have placed three 40 inch orders place one 120 inch order, which is the one you wanted. Keep the breaks few and whole-sheet (the whole sheet gets the lower rate once it crosses the line), because tiered pricing nobody can work out in their head does not change behaviour at all.
Fixed sizes, if you must
Some shops prefer a short list of named sizes because it simplifies cutting and stock. That is fine. Price each size by its area at your rate, so a customer who compares two sizes finds them consistent, and let the builder snap a layout up to the next size rather than refusing it.
What the builder has to do with this
Everything above only works if the number on the page is the number on the order. The length has to come from the layout the customer actually submitted, the rounding and the minimum have to be applied on your side, and an edited cart must not change the price. SheetGangster does the arithmetic on the server from the frozen layout, applies your rate, rounding, minimum and volume breaks, and checks every order against that figure before it can be fulfilled. The settings described here are the ones on its Settings page.