Is there a formula on how wide a sashing should be for the size of the block?

It should be pleasing to YOUR eye. I have been told that sashing sizes should be even divisors of the finished block size. Frankly, I don't set much store by that "rule", but often do follow it. For example, 8" blocks 'should' have 1 or 2" (finished) sashing. For a 12" block, 1", 2", 3" or even 4". 4" is just too big for most purposes. Tip: put your intended sashing fabric so it sticks out from behind a block. Slide the block around until what you have works to your eye. Once in a while I do pieced sashings. Those are an entirely different situation. The pattern needs to work with the block pattern. Storm at Sea is an example of this type.

Sizing a sashing is completely subjective. Do what works for the look you want. I do suggest you audition several sizes before making any final cuts.

QM is correct. There is NO rule for the width of the sashing. Just lay out the fabric you are going to use for sashing and put 4 blocks down and take photos of 1", 1.5", 2" or any other width between blocks and take a look at the photos on a computer or pad to make a decision.

I usually experiment with different size sashings in EQ. I can get a good perspective there of what looks good to me. And what looks good depends on many things - size of block, color of blocks and sashing, design of block, whether cornerstones are used, my mood at the time, etc.