I’ll expand the resizing procedure to cover any resizing/rescaling. The amount you need to increase or decrease the size is the ratio of the two scales. So, you make a fraction of the two scales- that is, writing one scale on top, over a line and the other under the line. That is the question- which is the denominator and which is the numerator. Here is a simple rule.
In making 1:48 into 1:72, are you making the decals smaller or larger. Obviously 1:72 is smaller than 1:48, so the ratio must be less than 1. The only way that happens is to divide 48 by 72, yielding 0.666. So you rescale by 66 or 67 %. Most photo editor software allows you to rescale by percent.
But suppose you are trying to make 1:72 decals into 1:48 scale. Here you are trying to make them bigger. So the ratio must be bigger than one. The only way this happens is if the top number is larger than the bottom (numerator larger than denominator). So 72 needs to be on the top and 48 on the bottom, or 1.5 (150 %).
This works for any two scales- if going bigger, divide the bigger number by the smaller on- if going smaller divide the smaller number by the bigger one.
I printed the decals on my inkjet paper using Kodiak brand decal sheets. I sprayed the with 3 light coats of Tamiya TS-13 clear spray and let them dry thoroughly. But when I tried to apply them I couldn’t get the decals to slide off the backing paper.
I use a spray box on my apartment balcony to spray decals and model parts. Before I sprayed those problematic decals, I had tacked down one side of the small sheet — it was only a small rectangle about 1” X 3” — with a short piece of scotch tape because the pressure of the spray sometimes moves what I’m spraying. I didn’t realize it at the time, but I was not only spraying the top of the decal sheet, but “overspray” was going underneath the decal sheet and making the backing waterproof, which made it impossible for water to get to the adhesive of the decal no matter how long I soaked it.
Last week, I tried again, but sealed the decal sheet to the bottom of the spray box with tape on all four sides, let the spray dry for about two days, and sat down to apply the decals again. Voila! Those decals slid off the backing paper with ease after only a couple seconds of immersion.
Note to self: Do NOT spray the edges or the back of decal sheets!