it is usually easier to just go ahead and cut a the next 1/8" up---so in your case- of needing 3/8" if you cut 1/2" you are just a tad over- and can then trim to size after constructing
but yes- if your ruler has 1/8" marks then 3/8" is one mark over 1/4"- right in between the 1/4" and the 1/2".
many patterns will also call for 7/8" which is just one mark less than the 1" mark- and again- if you cut the inch you have a little to trim (fudge room) which can come in handy if your seams are just a little (off)
1/8" may not seem like much- but---in 8 seams it equals a whole inch! so remember- it does add up- quickly and it does make a difference between blocks that match up and those that do not