I know this may not be what you want to hear but I have been where you've been and you may just have to get the ruler out and jot down some measurements. I have been a Stampin' Up! demonstrator for years and finally began writing down measurements on stickies that I put on my personal card samples. All you need to do is do it once but be sure to put those measurements in a place you'll remember, LOL!
For instance, as Roxy said, if you start with an A2 card or "quarter" card, we know that the card front is 4 1/4" x 5 1/2" no matter if you are using it horizontally or vertically.
How I start is to measure my first mat (whether it be card stock or background paper) at 1/8" narrower on EACH side. Keep doing that and you'll come up with something that looks like this if you were to mount each piece on top of one another:
Card = 4.25 x 5.5
1st layer = 4 x 5.25 and each layer would be 1/8" smaller on EACH side which means you deduct a total of 1/4" (1/8 + 1/
from each measurement (WHY does that second 1/8 have a smilie face with sunglasses??):
3.75 x 5
3.5 x 4.75
3.25 x 4.5
3 x 4.25
And so on...
Does that help?? Put some measurements on a Post It Note, tack it to your stamping area desk or at the bottom of your computer screen and use it for easy reference till you can do it automatically.
And have an alternate list with a larger mat. Let's do each one reducing by a total of 1/2" (1/4" on each side) for a different look...
Whatever you do.. HAVE FUN!!!!!!!!!
Card Recipes
