Hello. I am trying to create this:
http://www.clotilde.com/detail.html?...a=bobbin+saver

except in a straight version, not round. I have a piece of 1/8-inch
diameter silicone tubing and a piece of 1/4-inch diameter silicone
tubing.

My question is: Is there a way to calculate how wide I should cut the
slit/opening so that the piece will grip different sizes?
Any information about this will be appreciated.

Regards,
Amy