I agree with bj above - on one of Alex Anderson's Simply quilts show, a viewer asked your question and the response was to skip the 7/8" bit and just make the square a full inch larger than needed. It adds the extra step of cutting it down to size but it's no fun to get to the step where you square up your half square triangle and find that it's too small...
Thangles work well. I also have Triangulations CD by Brenda Henning that lets you print out a foundation for whatever size you need but this adds the cost of the CD, paper and the most expensive printer ink.
It is important to sew with a precise scant 1/4" seam but if you have tried and
are still having trouble, then the cheapest solution is to cut 4" squares to get
3.5" HalfSquare triangles and not 3-7/8".
Console yourself that there would not be so many products out there if this were as easy as it seemed to be before you tried it.