I'm trying to complete my beautiful Pascha basket pattern, but I just can't see the 30ct linen. I had bought groundcloth, but shortly after Krista started carrying white. All that said to ask for the conversion formula to get the additional yardage needed to switch from 30 ct to 28 ct. I feel like it might be multiply it by 1.15, but part of me doubts that and thinks 1.5
My first recommendation is a good pair of inexpensive readers--I always have to wear magnifiers to embroider.
But, if you'd prefer to switch to the 26ct fabric, then multiply the 30ct fabric by 1.20 to be extra safe.
Alternatively, you can take the stitch count, divide by 12.5 and then add 2 inches seam allowance to each side. For example, if the design is 245x245sts, 245 divided by 12.5 is 19.6 which is the actual size of the embroidery. Add 4 inches for the seam allowance (2 inches per side) and you'll need a piece of fabric that's 23.6 x 23.6 inches.
I'm using 12.5sts/inch instead of 13 because the white TG has warp threads that can vary, so I'm being extra safe.
Hope that helps!