I don't worry about 3/8" -- I learned my early quilting watching Eleanor Burns and so I always over cut HST and other triangle blocks. I round up to the nearest half inch (I always buy at least an extra 1/2 yard of fabric when doing a pattern that does hst's or other similar blocks) and then I trim as needed. I prefer to trim than have a block too small because I did not see the 3/8 or 7/8 line correctly.